So, to start with, this is about adding a link into the PM that gets sent to the group owner when someone requests group membership, right? The link should lead to the respective group’s page (groups/<groupname>), open the “add members” modal and ideally also already add the new users username into the modal so that the group owner only needs to press “+ Add”.
The last bits seems to be the difficult part:
is there a URL to navigate to the modal or would it have to be implemented as well?
even if I can open the modal with a URL, I suppose that there is currently no way of adding a username to that URL, right?