Can't add alternate hosts by username

[bjausel]

All other users are in the same organization and are under a paid account.  When i go to type in an alternate host, a a drop box comes down with suggestions to click in order to use usernames.   For example if i go to add "Tom Smith"   I will type T..o.. and then the auto complete will show me:  Tom Smith  (***********)   or Tom Sams (***********)   but not all the toms will be there.     

What is happening right now is that I would type Tom  into the box and Tom Sams would show up but not the Tom i would be looking for and just typing in his name gives me an error of  "Unable to assign "Tom Smith" as an alternative host because the user cannot be selected at this time".   But if i put in his email address of ***********, then it goes through fine.       

Does anybody know what it is that distinguishes when a user is used in the drop down box?    Everybody looks to be set up exactly the same, so i am unsure why i can't just find everybody there.