All available groups will be listed in alphabetical order. Select the one you want and click Join. If the group is public, you'll see a confirmation message and become a member right away. If the group is private, type a reason why you want to join > Send.
TIPS: How do you know at a glance whether a group is private? It's noted at the top of the group card, as in this example.
Once you've joined a group you'll receive a welcome message that contains important information and links to help you get started. If the group is public, you'll get the message right away. If the group is private, you'll get the welcome message after you've been approved by group owner.