Can’t add a Member to a Group

I have several members of my team that I’m trying to add to our Group. For a couple of them, I can find them and add them. For a couple of them, I can’t find them when I search on their name or email alias. They’re in Outlook, but I can’t find them in a search. Any idea of what I can do to fix this?

I asked this in another section, and was asked for more details:

Selfhosted / instance The URL starts with https://sfgitlab.opr, but has my company URL following this
Authentication method (LDAP?) I use my LDAP auth to log into the app, but don’t know what auth is used to find members to add to the Group