Are there RAW sources for these modifiers?

[Pursuivant]

I'm trying to design some powers which need enhancements or limitations I don't recall seeing before - either officially or unofficially.

Ceremonial: Your power works like a ceremonially-cast magic spell or an enchantment using the standard Magic rules. Others who share your power can contribute FP to energy costs to power it, suitably motivated and qualified assistants who don't share your power can contribute 1 FP, and those assistants who are hostile to the use of your power effectively subtract 1 FP. Generally must be combined with the Preparation Required limitation.

Strengthened With Cooperation: If you can combine your power with others who share your power, the members of the group either get bonuses to effective skill to use their combined powers or else the effects of their powers are somehow increased. Possibly a trait such as Mindlink, Special Rapport, or Telecommunications is needed to coordinate the activity. Essentially like a Psionic Gestalt.

Strengthened With Preparation: The opposite of Preparation Required or Weakened Without Preparation. Taking extra time gives you a bonus to use your power or somehow enhances its effects.

Time Sensitive: Essentially the opposite of Preparation Required, but only suitable for traits which are used in reaction to some prior event or action. For example, using Detect to sense traces of something which was previously in the area being analyzed or using Healing to treat a wound long after the damage was inflicted. It could use Geographical/Temporal Scope modifiers as penalties to skill to successfully use the power, or else the effects are somehow reduced based on the amount of time which has elapsed prior to its use.

[Varyon]

Originally Posted by Pursuivant

Strengthened With Cooperation: If you can combine your power with others who share your power, the members of the group either get bonuses to effective skill to use their combined powers or else the effects of their powers are somehow increased. Possibly a trait such as Mindlink, Special Rapport, or Telecommunications is needed to coordinate the activity. Essentially like a Psionic Gestalt.

Pyramid #3/65, the "Team Up!" article, has the option of building a power that requires n characters to use - simply divide the point cost by n. To have this strengthen an existing ability, simply buy the normal version of the ability, then apply this treatment to the upgrade. For example, if you have two fire-wielding supers who can combine their flames, each might have Innate Attack 4d Burn [20] + Innate Attack 4d Burn (Requires Teamwork x1/2) [10], such that they can work together to create an 8d attack. You could technically have 4 - or more - characters with such powers, and can mix up which two are contributing. Alternatively, buy the two versions as Alternate Attacks (which may be a better bet - above, Innate Attack 8d Burn (Requires Teamwork x1/2) [20] is only [4] as an Alternate Attack). You may also get some mileage out of Combining Powers, found in Powers p160 and the above article, and the various teamwork options from the article.

Originally Posted by Pursuivant

Strengthened With Preparation: The opposite of Preparation Required or Weakened Without Preparation. Taking extra time gives you a bonus to use your power or somehow enhances its effects.

See Partly Limited Abilities (PU8:6), which is also involved in the approach I took above. Basically, just apply Preparation Required to the levels of the ability that need it. A fire super with a 4d Burning attack that gets boosted all the way to 20d if he spends 1 minute "powering up" beforehand (screaming, forehead veins, and flashy auras optional) has Innate Attack 4d Burn [20] and Innate Attack 16d Burn (Immediate Preparation Required, 1 minute -30%) [56].

[Refplace]

Also rules for combining powers are in GURPS Powers.
As for ceremonial, that would include extra time but the rest? Not sure requires pondering.

[Anaraxes]

A standard magic mage has to meet a few requirements to be able to contribute to a ceremony (Magery, skill 15+, etc). So a magic-as-powers analog might require an Advantage from a contributing caster -- say, "Affects Others" on the mage's ER, along with a suitable Accessibility (or Limited Enhancement, if you prefer) to say "only during ceremonial casting".

If you want the energy-tapping to be a cost of the mage leading the ceremony, then you might build it as Leech, "Only from willing targets during ceremonial casting". If you want to get really fancy, you could build it as a levelled advantage to reflect the 1 / 3 / unlimited thresholds, with the Accessibility (or Environmental) Limitation on each level reworded appropriately (any spectator, mage or skill 15+, mage and skill 15+), and of course increasing in value at each level.

But I'd start with the Combining Powers rules on Powers p170.

[Pursuivant]

Originally Posted by Varyon

Pyramid #3/65, the "Team Up!" article, has the option of building a power that requires n characters to use - simply divide the point cost by n.

Those options sounds cool, but rather than a Voltron-style combined advantage which doesn't work unless all the team members cooperate to make it happen, I'm thinking of an advantage which works just fine when used solo, but gets boosted if you have friends to assist.

Cooperative advantages and Partially Limited Abilities make sense for leveled advantages where teamwork can boost effects, but not so much for an advantage which normally requires an attribute or skill roll.

Specifically, I'm thinking of the Healing advantage, where getting an assist from fellow healers boosts the lead healer's effective IQ to treat disease or other ailments which carry a skill penalty.

[Varyon]

Originally Posted by Pursuivant

Specifically, I'm thinking of the Healing advantage, where getting an assist from fellow healers boosts the lead healer's effective IQ to treat disease or other ailments which carry a skill penalty.

Figure out what it would cost to have the bonus from the start, then divide that additional cost amongst the participants (or build it as an Alternate Ability, if that's cheaper). So, let's get a little complicated and say you build Healing where each additional Healer - up to 5 assistants (6 people total) - gives a +2. As an AA group, this is Healing [30], Healing (Reliable 2 +10%, Requires Teamwork x1/2, AA x1/5) [3.3], Healing (Reliable 4 +20%, Requires Teamwork x1/3, AA x1/5) [2.4], Healing (Reliable 6 +30%, Requires Teamwork x1/4, AA x1/5) [1.95], Healing (Reliable 8 +40%, Requires Teamwork x1/5, AA x1/5) [1.68], Healing (Reliable 10 +50%, Requires Teamwork x1/6, AA x1/5) [1.5]; for a total of [+10.83], rounded up to [+11] and a total of [41], for the ability to incorporate teamwork (note it's [+13], total of [43], if you follow the RAW route of rounding each component up before adding them together). To do it simply as Limited Enhancements, it's Healing (Reliable 2 (Requires Teamwork x1/2) +5%, Reliable 2 (Requires Teamwork x1/3) +3.33%, Reliable 2 (Requires Teamwork x1/4) +2.5%, Reliable 2 (Requires Teamwork x1/5) +2%, Reliable 2 (Requires Teamwork x1/6) +1.67%) [34.35], rounding up to [35]. In this case, Limited Enhancements is cheaper, so go with that. Note that extra [0.65] remaining is enough for +2.17% worth of Enhancements, so you could toss on Reliable 2 (Requires Teamwork 1/7) +1.43%, and optionally Reliable 1 (Requires Teamwork 1/8) +0.625%, at no additional cost.

For abilities with built-in FP costs, like Healing, I'd personally have no objection to characters working together in the above fashion to be able to split the FP cost amongst themselves. It should probably be split as evenly as possible (perhaps to the point of needing to round up), but that's up the GM.

[Raekai]

I'd also go with the rules for combining powers in GURPS Powers.

Hidden in the same book, spirit powers get ritual length as a special rule on p. 178.

Originally Posted by GURPS Powers, p. 178

Ritual length: To negate the above penalties, the sorcerer can take extra time to use his ability. Shifting a time requirement of seconds to minutes, one of minutes to hours, or one of hours to days gives +3; two steps give +5; and three steps give +7.

This doesn't come with an associated cost, unfortunately, though it is balanced by the penalties that spirit powers also carry for affecting others, consecration, and regalia. Of course, just above, it also says that the GM may apply "some or all" of these modifiers.

Either way, that's what I use. It's a nice technically RAW solution, and I've called that bonus for taking extra time a +10% enhancement, though that's a bit dubious, and I would be more than happy with other prices. It'd make sense to guess what the limitations are worth and price this enhancement accordingly. I'd ballpark affecting others at -5% or -10%, consecration as -5%, and regalia as -5%. That'd probably make ritual length closer to +15% or +20% as an enhancement.

Aaaand that's only useful if you think that might be a good solution for what you're trying to do, so I'll stop my rambling.

[Anaraxes]

Bonuses to casting skill rolls thanks to a ceremonial group can be handled with the Basic rules on B342 (Takes Extra Time, Long Tasks).