Hello,
please provide more info, what users are they, global or not.
If its not global user, then it cannot have access to company, except if user is created in Person register. If user has to have access to only one company then you define user only in particular company. If its a global user then it can access to all companies.
More info here:
http://www.hansamanuals.com/main/mailnumber___6042/theconf___581/manuals/levelzero___6042/version___54/hwconvindex.htm
Regards,
Tomas