OK, this is going back to earlier - but I think it's VERY clear from the movie that Angel knew Mimi before she met Roger. After Rent, there's a shot of Angel looking up, pointing his drumsticks. During the funeral, Mimi definitely sounds like she's known him (which explains the Halloween reference in her speech). Therefore, Angel coming to sing with her, seeing a friend who's CLEARLY hurting, makes sense. Marc and Collins are great friends with Roger - Mimi is imploring him to come out, Marc's been trying this for all this time, he KNEW who Mimi was ("the stripper from downstairs"), so what's wrong with them coming and singing with her at the end of Another Day?
Hi, Shirley Temple Pudding.