Must use lower case letters or numbers only.

made a new site and tryed word press instead of what i normally use, emailed the group to join and getting emails back that persons cannot join! What they put down as user name, thier name, very first thing you fill out with password always gives error, ( must use lower case letters and numbers only ).. this is being done, what is not being done is site is made and usless now as so many cannot even join the site? did search to see if anyone else is having same problem.. I made up a fake name with another email of mine and it worked fine, wife did it in front of me and had the error so it is real. site is http://www.soulguildes.net ty now for any help! :+) ps..