Adding Users to a Workspace

I am trying to create a Shared Workspace, but when I try to add users, my users are not showing up, I cannot add anyone. Can you have users in multiple Workspaces?

A co-worker of mine was trying to do this, and failed. So as the admin, I went and tried it out, and got the same results.

What am I doing wrong?

Need help ASAP.

Anthony