A couple things to keep in mind. Ritual casting covered starting on page 295 of the core.
This example is real bad. Ok. So Let cover basic things, the limit of the force of the ritual is equal to rating of the lodge you casting or, you can use a number of drams equal to the force of the ritual you want to make a temporary lodge. Next for combat spells your using prodigal spell ritual which btw takes you force in hours to complete. Timing that exactly would be a very impressive feat, but let assume they figured that out. Next. You do the ritual casting (see 295+), dropping at 12 drams of reagents probubly a decent amount more to lower the drain. Then the Seal check or completion check of the ritual is Ritual Magic+Magic[force] vs Force x2, now you do get a teamwork test for each additional participant. But if you do the math on this very quickly. Our chick has magic 6 and we assume ritual magic 6 plus a power magical focus lets say 4, so that's 16[12] vs 24 dice, now lets say her new boy friend is also a professional ritual spellcaster and they are actually using a spirit as a spotter. So he's got all same stuff she has, and we will assume they share the same tradition. So he has the same 16 dice which of which average gets you 5 successes which passes 5 dice on too his GF. (See page 49 of the core for team work tests). Ok So that puts us at 21 vs 24 dice. So if we play strict averages 21 dice is 7 and 24 dice is 8 thus ritual fails, and all the drams of reagent spent and 12 hours of her and new bf lives are wasted but the even worse news is they now roll 24 dice and suffer drain equal to hitsx2 x-reagents spent on drain Minimum 2 which in this case is stun b/c he passed her 5 dice and that is lower then her magic of 6.
So a couple common sense points about this.
Given the constructions of rituals unless you and your crew are the grandmaster of rituals lets keep it simple. Use spells where min effect will handle the problem. In this case indirect is probably better. Given ritual casting works spell drain is fairly irrelevant and you can spend whatever money you need to drop it to 2. So lets keep the same scenario lets serously mess up one single guy. So lets say 9 health boxes, and will use Flame Thrower in place of stunbolt and lets also say we know he will be in bed with his new girl, so we know he won't be rolling around in lot of armor, he won't see it coming, and there will be a window open. So now try it again. So magic lodge rating needs to be force 9 which will also be the force of our ritual. 9 hours later 9+drams of reagent spent, assuming everything goes the same, we are now looking 21 vs 18, so averages say 7 vs 6. This launches our force 9 flame thrower spell at our target. Damage is 10 assume he's got a body of 3, he suffers 9 damage and may or may not be is dead before doc wagon can get there. Regardless he had a really, really bad night, not mention those poor, poor Egyptian cotton sheets.
This example is also far from perfect, we made a decent list of assumptions the biggest being an open window, but if we skip getting to specific on the time, and just settle for when the target is asleep, and we take some steps to ensure a window will be open. It's probably a workable scenario.
To answer specific questions:
Everyone in the ritual suffers drain, which is the hits from force of ritual x2, hits x2 less reagents spent on drain, If the number of hits the leader got on her
Teamwork Test was higher than her Magic rating, this drain is Physical; otherwise it’s Stun. In this case its stun. In this case that's 12S drain - reagents spent on drain min 2.
That includes the spotter who doesn't act in the teamwork test.
The question that comes to mind, is what happens when you edge the sealing test. Theoretically cast something like stunbolt with a force of 1, edge the roll, and then pummel the other side with successes. Say an edge of 4, 25 vs 2 using stun bolt, something like 12 successes. Drain basically min and you did 12 vs will stun to guy, and it only cost you 1 reagent a ritual area force 1 and an hour to do it. There in lies the danger imo. Now hopefully I'm missing something and someone can explain why that won't actually work.