In the Social Workspace, you can find your Groups. Joining a group is very simple!
Click on the nine dots in the bottom and head to the Social Workspace.
Select Groups -> All Groups from the navigation menu.
Select the group you want to look at. Private Groups are only available to the group creator and invited members. Public groups can be seen and joined by anyone, as long as they have read permissions for the category the group is in.
Joining a Group
When you have selected the group you wanted to join, click on the Join Now button.
Requesting to Join a Group
If a group needs permission to join first, there will be a Request to Join button. In case you wish to cancel this request you can select the Cancel request option.
As soon as the Group's admin accepts your joining request you will be able to participate in the Group's activities.