3. Add that group as a member of your Place
Are you referring to a "Group"? If so, I do not know of a way to mirror access to a Group, with a User Group aside from an API call to GET the users of the User Group then looping through your users with a PUT request to update the users of the group.
Hope that helps,
I think at the add ons list, if you go to the settings, it asks you an option to add security groups. Create a security group by going to /admin > groups. Add members to the group. Now,
Add ons > settings button (on the right of your app) > here you can add a security group you had created earlier to make the app visible to the specific members.