In my opinion members lookup (in “Add Member” pop-up at the Team page) should show only users, which doesn’t belong to the current team yet. It’s very annoying, you can’t simple check who else should be added to the team, because lookup lists all available in the system users, including these already added to the team.
I’ve just tried this myself and I can completely understand why it might be frustrating to have to search through users who are already on the team. I’ve therefore raised this as a potential improvement for our product team to consider.
I hope this is helpful. Please get back to us if you have any questions.