Joining a group
Click on "Groups" to see a full list of all groups, or use the search box to search for a group you want to join, and then click groups to search for groups. Is the group you are looking for not existant?
Create it!
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Click the group you want to join, and then click "Join group".
This will send the group owner a request to join. Once you get approved, you can start posting in that group. For public groups, you do not need to be a member to see the posts.
If i get invited to a group
If you are to get invited to a group, you will see the invitation in your notifictions. Click the invitation to visit the group.