Allow only global admins to directly add users/groups to a workspace

The idea is that any user that is able to create a workspace can also forcefully add users/groups to that workspace. In an open environment, this might create problems with users being added to workspaces without their approval.

A solution would be that regular users should only be allowed to invite people to a workspace and only admins should be able to forcefully add users/groups.

This applies also to the workspace creation page.