Hi,
Only a guess but perhaps the settings for team-name is restricting the ability to add members to projects directly. I see no other reason for this otherwise.
On gitlab.com I maintain my own private repos, and I can invite people to them, but I do not have a team configured. This is why I think the team is the problem.
this is for a group I have test-group, in the group settings and the section under General → Permissions there is an option to restrict adding project members. Maybe you need to check if this is enabled and most likely disable it.