Check users on roles when adding them to groups

Problem/Motivation

When adding people to a flexible group that is restricted to specific roles, people without the right role could be added to that group.

Steps to reproduce

- Create a flexible group that is restricted on role.
- Try to add a user (Add directly) to that newly created group that doesn't have the role.
- The user is added to the group, which is not allowed

Proposed resolution

Add extra validation to check if the user is allowed to be added.