How do I get what's in the following picture (from the gvectors addon site, also runs wpforo so). I'd like for my users to pick their own secondary user role but for me to choose from which user roles they can pick e.g.: my site as role a, role b and role c. The user can only pick role a and b, role c needs to be added by an admin.
You need to Select which usergroups can be shown as secondary in Secondary Usergroups Field,
Go to Dashboard > Forums > Member Fields, edit User Groups Secondary field and check Display Usergroups field.
Also, please note: all questions related to wpForo paid add-ons should be asked at gVectors.com community. Here we support issues related to wpForo