To find a group select the green circle with white + in the lower right hand corner of the page. Then select Find Group.
At the top of the next page, you’ll see a grey bar that says “Search”. Select inside of the grey search bar. Type in the name of the group you want to join and then select Search. From the results that appear, select the Join option on the correct group. If the group has an open privacy setting then you will be added right away. If the group has a close privacy setting, a request will be sent for the admin to approve you joining the group.