Have you ever been in a jam session, and were taken by surprise by something? Maybe the other pickers asked you to play or sing a song. Or perhaps while the group was playing, you suddenly had the tune passed to you--and you didn't know what to play!
If you're new to playing in jams, things like that can take you by surprise. If it's all you can do to watch a guitar player to find out the chords, figure out where they are on the banjo, and then vamp or play some simple backup, it's hard to do anything else at the same time--such as think about a break to play before it's your turn. But you can have a plan of action.
Think ahead, and know ahead of time what you're going to do. If the chords to the tune are pretty familiar and you can use some of your familiar Scruggs licks to build a break, start planning for that as soon as you have the chords figured out. If, on the other hand, you don't know the tune and don't want to make a leap into nowhere with your banjo break, just tell yourself ahead of time that if the tune gets passed to you, you'll just nod to the next person and pass the break off before it's time to start playing. Whichever you do, the tune will go on smoothly, and you'll be more confident and better prepared for the next time.