GarthMJ's report gives a nice list of machines and their last logon user.
Here is one that will give the machine, all logged on users, and when they logged on.
Select
V_GS_SYSTEM.Name0 as [ComputerName],
V_GS_NETWORK_LOGIN_PROFILE.LastLogon0 as [Last Login Time],
V_GS_NETWORK_LOGIN_PROFILE.Name0 as [Logon User],
V_GS_SYSTEM.Domain0 as [Logon Domain],
V_GS_SYSTEM.SystemRole0 as [system Role],
V_GS_SYSTEM.SystemType0 as [system Type]
from V_GS_NETWORK_LOGIN_PROFILE
left JOIN v_GS_SYSTEM ON V_GS_NETWORK_LOGIN_PROFILE.ResourceID =
v_GS_SYSTEM.ResourceID
where V_GS_NETWORK_LOGIN_PROFILE.LastLogon0 is not NULL
AND
V_GS_SYSTEM.SystemRole0='Workstation'
Order by V_GS_SYSTEM.Name0
