Public groups are available for all users that belong to the set audience. Any user within that audience can click to join the group.
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Private groups require an application. To join a private group, the user needs to click Apply. This Apply action may request the user to fill out a quick application form limited to three questions. The group admin will either approve or reject the application.

If the group has specific requirements or an approval process to become a member, it should be set as a private group. Otherwise, set the group as a public group for users to be able to join without an approval process.