Becoming a member of a group is a good way to be a part of a movement or cause that you care about. Joining may give you access to special events that would otherwise be blocked or invisible from the general population.
By default, every group will have a button to become a member or join the group. This button is displayed below the title of the group's name. If you are have joined or have become a member, this button would not appear but be replaced with the words "Member".
When becoming a member, depending on the membership requirements set, you may join immediately or be requested to fill out fields (essentially an application) and it would require you to wait for approval until the admin or organizer approves.