When you register a new user it doesn't allow you to use certain characters in the username, however it is possible to use those characters when you rename a username. For example, we prefix all tester accounts with "[Tester] " so we know when a comment or something else has come from a tester. The steps I have to perform are:

Create user
Username = "Mr X"
Activate user
Manage user
Username = "[Tester] Mr X"
Save changes

If there is a good reason for not allowing "[" & "]" then stick with it when renaming a user, but if not, allow it when registering a user for the first time.