Group Accounts version 2.1 introduced a new feature called "Group Member Registration". This option can be enabled in Restrict > Settings > Misc.
Previously, the only way for a member to join a group was for the group owner (or admin) to fill out a form to invite a user. With Group Member Registration enabled, group owners can instead share a unique URL that leads to a public registration form. Anyone with this URL can sign up to join the group without needing a manual invite from the owner.
In order to utilize this feature, you first need to create a page that will serve as the public registration form. Create a new page via WordPress Dashboard > Pages > Add New and insert the shortcode [rcp_group_join_form] somewhere on the page. Then, go to Restrict > Settings > Misc and select this page as the "Group Join Page". Also, make sure you have "Allow Group Member Registration" checked on.
Groups have to opt into this feature by setting a group "code". This can be done via their group dashboard.
Once saved, a new section will appear in their dashboard titled "Invite Group Members". This provides the unique URL they can share to invite people to join.
Site administrators can also set codes via the Group Details page:
Public Registration Page
When someone visits one of the unique invitation URLs, they'll see a form like this:
The "Group Code" field will be automatically populated from the unique URL.
Once submitted, the user will be given a new account and will automatically be added to the relevant group (provided there are remaining seats!) with the role "member".