Yes, I don't think anyone will mind you taking part in group threads, we often pop into a group other than our own if we find the thread of interest.
What you won't be able to do, is read work that is uploaded for the group members to see only.
But, why don't you join a group, then you can comment on people's work and maybe upload some of your own for comments. It'll give you a good idea of how things work then.
Welcome to the site by the way