Sometimes autistic people want to hang out with a group of people and can’t tell if it’s ok to talk to them or not.
And then, they try to watch for a while to try and pick up on signs that interaction would be welcome.
This is generally a bad idea. The problem is that people find it uncomfortable to be watched by people who aren’t explaining their presence. If they’re not ok with you joining them, they’re probably not ok with you watching them, either.
It’s better to just go up to people and ask if it’s ok to join them. Watching first for more than a couple of seconds actually makes things worse.