If you have a public group, anyone can join your group. 

If you have a private group, anyone can apply to join your group. 

If you have a secret group, only members who have received an invitation can join your group. 

You can also filter the type of profiles you want to have in your group, and thus restrict memberships to certified members, members whose profile is more than 10 days old, couples, single men, members with a photo, by geographical area, etc. 

To do this, go to the "administration" tab of your account and then to the "adjust group filters" section

Web version : 

Mobile version :