Joining or Leaving a Space

Users can join or leave a space any time.
However, if "Block users from joining or leaving the space and following or unfollowing the threads." option is enabled in the "Space Settings", users cannot join or leave the space themselves. If that is the case, only the space administrators can add or delete space members.
Refer to "Adding or Deleting Space Members"

Joining a Space

To join a space, open the space that you want to join, click the Options button Image, and then click Join Space.
When you join the space, you are automatically set to follow any threads that exist within the space, thereby allowing you to receive update notifications. Additionally, the space members are displayed in "People" section in the Space Portal page. Picture

Leaving a Space

To leave a space, open the space that you want to leave, click the Options button Image, and then click Leave Space.
Leaving the space would also lead to your unfollowing any of the threads you belonged to within that space. Additionally, once users leave a space, they will no longer be displayed in the "People" section in the Space Portal page. Picture