Can we change the user info in the default registration form?

When some users register on our site, they put their full name with capitals and spaces in the username field like : "Sam Smith".
This is changed by WP to "samsmith" without telling the user which confuses some users.
We need at least to put in som explanatory textfield next to that but can't find where? We are using the plugin "Invite anyone" which lets us change all registration fields except this one.

Any simple way to do this?