Organisationgroup: option that added members can leave themselves

Context / Problem: If you make a public organisationgroup, you need to explain people how to become part of the group, however this often has the that the group is not found or little people join.

Desired solution:
- Keep the existing group type (public group)
- Add an setting where users are added, but can leave the group themselves afterward.

The additional option provides that flexibility alongside the existing group type.