Add the following variable to sametime.ini and stop and start the Community server.
This will enable LDAP trace and add stresolve*.txt, stdirectory*.txt and stuser*.txt files to the \Trace directory. The problem you face may be LDAP related. Often poorly configured LDAP settings means that users and groups cannot be retrieved using the search filters detailed in the LDAP document in stconfig.nsf.
The stresolve*.txt files will help. Look out for messages similar to the following.
100916_112422.006,FTL,LDAP Res,Warning: pending operations not responded within timeout
100916_112720.543,FTL,LDAP Res,Number of pending operations dropped below 30. Resuming to send requests to LDAP
You should also consider the following settings which will help.
ST_DB_LDAP_MIN_WILDCARD=<Minimum number of characters>
ST_DB_LDAP_MAX_RESULTS=<Maximum number of results>
Also look to whitelist your SMTP domains so that @hotmail.com and other external addresses aren't sent to your LDAP to resolve.
The following document is useful.