What is a squad?

And how do I get involved? The app says it will assign me a to a squad but it can’t seem to work out how that happens and if I have been assigned how do I access it? Is it a sort of self-help group? Do I have any choice about how and to whom I’m assigned? And do the others have any choice about me being assigned to them? It’s all very opaque to me right now, though I do like the idea of joining a group of some kind. Any help/information gladly received! SS