For a band to work well, it has to be in sync.
The band is always managing a million things. They need to keep rhythm, make sure every musician is starting and stopping on time, playing at the right volume, that the audience is engaged - and that everything works together to produce an emotional musical experience.
If you want to join in, being proactive can make things worse.
If you just start playing, you're adding another person to manage. They have to work with you, get you in sync and in the right balance. This takes their attention away everything else.
If you want to join, you need to work around the most overloaded people in way that doesn't add to their load. You need to be available to jump in at their convenience. You need to get ready and wait.
So set yourself up, make yourself known and watch the band leader.
Listen to the beat and tap your foot. Make eye contact.
Wait for their signal, and then you start playing.
More people means more work and more complexity. If you want to help, make yourself available and ready.