How do I join a group?

I've joined at least two groups, so you'd think I'd remember how I did it, but I don't. I want to join the Peer Review Group but can't figure out how. I go to the group and see the message that says I must join to see the posts, but I get no further. Please advise.

Hiya, the 'join group' button is located in the bottom right corner of the group's header picture/banner.