A wikidot user usually signs his(her) posts/pages with his(her) user name like this : [[*user UserName]]
For example, [[*user gerdami]] would render gerdami
There are some difficulties to await with User-id's which have a space or Underscore in their name. If you need to use such user-ids - they where stored (by Wikidot UNix Name Convention) with hyphen instead of spaces and Underscore.
For example : User-id [[*user Helmuti_pdorf]] would render Helmuti_pdorf and is
not the problem, but storing it on any page like a "site" changes automatically the name to http://helmuti-pdorf.wikidot.com/ - with a hyphen!