Using the Settings window to add a new user yields two input boxes; typing an upper case letter - at least in the 'user name' as against 'full name' box - produces a little yellow exclamation mark and stops one from proceeding. There's no explanation given, so far as I can see, of what is causing the problem. I had to use the web (see http://superuser.com/questions/827538/not-able-to-create-a-user-account-in-linux-mint-17-qiana-with-mate-desktop - which is from some five months ago ). Dear me! What should happen: if uppercase is to be disallowed (even in a name?!), this should be made clear to the user - preferably before s/he choses a name, but, at the least, afterwards i.e. if the user breaks the rule in question.
I am reminded a little of Douglas Adams's 'beware of the leopard' joke.
I have Linux Mint 17.1, x64 Cinnamon (but the problem is reported, at the link I cite, on Mint 17 Mate, too).
The little exclamation mark is trying to tell you something - it has a tooltip.
The actual reason, you could probably dig up, has to do with the fact that some OS's linux can interact with are not case-sensitive, and may not realize you are, in fact, trying to connect with a username of jOeuSEr and not joeuser. It's not an issue within Linux at, all, but a policy decision not to allow it that we inherit (and more or less agree with) from Ubuntu.
Btw, I can't speak for Mate's UI in this, just Cinnamon's.