A frequently asked question concerning Active Directory-based networks is, How can I tell who is currently logged on to my network? Unfortunately Active Directory itself has no built-in functionality to determine this, but there's a free tool from Microsoft that can help in this regard. The tool is called LimitLogon.exe and its main purpose is to limit the number of concurrent user logins to a domain, but it can also be used to keep track of all logins in a domain.
LimitLogon is an unsupported tool and can be downloaded here from Microsoft. There's also a FAQ on Bink.nu that explains how the tool is used. As usual, use unsupported tools with caution and try them out in a test environment first before using them on your production network.