You can still respond on bugs, even if they are closed - I assume that still notifies the developer.
As to finding groups, you can search for things that interest you, like game titles, or other interests, where you live, etc. You can also look at your friends' groups, or potentially ask if there's a particular group you're looking for.
That said, what are you hoping to achieve by joining a group anyway? As I personally don't find a lot of use for them, since discussions in the group aren't notified to members, which makes it not very useful for things like trying to organise a game together. They're fine for restricting, but if you want to restrict to a group, one assumes you know the members who you want to join it (and often what you want is just restricting to your friends, which doesn't require any special group).
