haliparotin
SME server by design only supports one username set.
You should VERY CAREFULLY consider the ramifications of username selection in relation to domain styled pseudonyms, before you start creating usernames (ie in a situation where multiple domains are hosted on the same SME server).
What this means is for you to think about how you will name your users, & what email address they will use
eg
the username can be of the form fred1 & the corresponding email address would be of the form fred@maindomain.com
Another user who wants the fred pseudonym would be fred2 with email address
fred@myotherdomain.com
& so on etc.
Refer
http://wiki.contribs.org/SME_Server:Documentation:Administration_Manual:Chapter9#PseudonymsPlease read all of the above section, particularly the sections titled
Practical usage guidelines
&
Summary
&
Alternative configuration of users
For an abbreviated version of the above also see,
http://wiki.contribs.org/SME_Server:Documentation:FAQ:Section04#User_accounts