I see three main ways of handling this sort of performance:
- Abstract, a single roll to determine the result.
- Skill challenge, everyone makes a check and you total up the results.
- One-sided Social encounter.
I think the last one might be most what you are looking for, so I’ll discuss the first two (which are the simplest) first:
For abstract, it’s just a group Charm check (highest skill, highest characteristic, Boost if the same person has both) with a Difficulty set by what the group is trying to achieve, or Simple with Boost/Setback for receptivity if degree of success (such as a street performer hoping for money) is all that is required.
For a skill challenge, you have each character make a particular check based on what they are trying to do as part of the performance and pool the net Success and Advantage for your final result.
For a one-sided Social encounter, the crowd counts, for all intents and purposes, as a single entity. They are there to be entertained, and so are not opposing the attempted checks; instead, the Difficulty is set by what the PCs are attempting to accomplish and how receptive the crowd is (the crowd may occasionally attempt something like a Coerce check, with a pool set by proportion and vociferousness).
The band members can attempt various checks to positively affect the crowd, swaying them to the band’s side. Perception to deduce the crowd’s mood and interests, Vigilance to detect threats or mood shifts, Charm to perform a song, Coordination to dance, Athletics to play music (e.g., drummer), Mechanics for the pyrotechnics, lights, etc., Deception to tell a story (truthful or not), Discipline or Cool to keep steady in the face of a rowdy/hostile crowd, etc.
Maybe there’s a certain amount of time/number of checks they have to make as part of the performance (e.g., “three rounds”), but it could just as easily be fluid. The idea would be to have the crowd react dynamically to what the PCs do, and to give the PCs a range of options to handle said reactions. Certain crowds may have proclivities toward certain styles or music choices. For example, one alien culture may have a particular appreciation for ballads and stories, while seeing any love songs as inappropriate, while Zeltrons might have little interest in ballads and stories, but a great enthusiasm for sensual love songs. Knowledge skills during the performance or ahead of time can help the group plan for a more successful effort.
Your Advantage/Triumph/Threat/Despair chart could be translated to this pretty easily, with maybe a couple additions or adjustments like an option for observing mood/gaining information or narrative results like gaining a fan/fanbase, making a name locally, etc.
This is a cool premise and was a lot of fun to think about, thank you. I hope this is helpful.