Prioritize Organisation Users in invitation modals

Problem

On GitLab.com, when inviting a new member to a group/project, we currently list all users of GitLab.com. It would be more user friendly to restrict these invitations to Organization Users and/or prioritize them in the invitation modal.

Proposal

Here are some options that we should validate further:

• Prioritize Organization Users when listing users in invitation modals for groups and projects. Perhaps they could be listed first as Organization Users followed by GitLab.com Users.
• Restrict invitation modals to display only Organization Users altogether, for example via a setting. For Organizations that want to ensure that group and project Owners do not invite external users, this might be desired.
• Let users select while working in the modal whether they want to focus only on Organization Users.