New users can join public groups as they register.
- Go to Users > Registration Forms, edit the form, and open its Settings.
- Turn on Allow Users to Join User Groups (the name follows your group name; see Changing the name of the group object).
- Save.

The form then lists your public groups as checkboxes. The groups a new user ticks are joined straight away when they register.

Mentioning the groups in emails
The {joined_groups} tag lists the groups picked on the registration form, for example in the registration confirmation email.
For developers
wpum_groups_registration_form_field_labelfilters the field’s label.wpum_groups_registration_form_field_descriptionfilters the text under it.
(Before Groups 1.3.0 the description filter also changed the label. If you used it for the label, move that to the label filter.)
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