[Bug]: Regional admin cannot invite admin to region that's already an admin elsewhere

[AI7NC]

Platform

MeshMapper Web

What happened?

I'm an admin for EUG, there was an attempt to invite me to another region but the regional admin add interface wouldnt' allow it as 'the user already exists'.

How to reproduce

Go to a region an admin username isn't allowed for logged in as an admin of that region. Click on admin tab, then add administrator. Add that users username and contact info. You get an error that that username already exists.

What should happen instead?

Even if username already exists due to roles elsewhere, the region should just be able to expand the role of that credentialed user to the local region.

How bad is it?

Low - Minor issue

Anything else?

No response