Putting the obvious plot reasons aside, I'm sure everyone have had an experience where your friend/s doing something awesome and you're waiting for them to ask you to join, because you don't want to seem like a bother or get in the way, and they never ask you because they don't know that you're interested and they don't want to bother you. It happens, and like Sunset Shimmer said in the movie, she didn't want to speak up because she didn't feel like it was her place, being new at "the friendship" thing. During her night conversation with Twilight she basically screamed "I want to help so badly but no one is asking me"
Also, the Rainbooms might've though Sunset Shimmer would be too uncomfortable in the spotlight, which was probably true at that point. Even if they would've asked, she could very well have turned it down