Self-Signup Option for Private Communities

In a private community, to gain access typically users will need to invited by you or gain access through Single Sign-On. Alternatively, you can enable the option for users to self-sign up, and moderators can review and approve their request to join.

This option is available Admin > Settings > Moderation: 

When enabled, users can request access by entering their details, including any required custom profile fields you have defined.

Moderators are notified and can approve or reject (block) users:

After approval, the user is sent an email letting them know their request is approved.