If you need to find out who currently has an open session on your server via T-SQL in SQL Server 2008 you can use the 'Dynamic Management View'; sys.dm_exec_sessions
Example code for sys.dm_exec_sessions:
The WHERE clause ensures that system processes are either included or omitted.
sys.dm_exec_sessions returns one row per authenticated session on SQL Server.
Further Reading:
List user processes:
select *
from sys.dm_exec_sessions
where is_user_process = 1
List system processes:
select *
from sys.dm_exec_sessions
where is_user_process = 0