for the unique usernames, you could search for usernames and then pipe to the 'uniq' command:
http://www.splunk.com/base/Documentation/latest/SearchReference/Uniq
for the count of users logged in, you can use the stats command with the count function:
http://www.splunk.com/base/Documentation/latest/SearchReference/Stats
and
http://www.splunk.com/base/Documentation/latest/SearchReference/CommonStatsFunctions
you might find the Splunk tutorial helpful in general:
http://www.splunk.com/base/Documentation/latest/User/WelcometotheSplunktutorial