In what respect? Terminal Services? Shared Folders? SQL Database access? Other?
Yes, Yes, Sort of, Maybe.
In terminal services, open terminal services manager and it will list all connections and who has them.
For shared folders, open Computer Management, and under System Tools, expand Shared Folders. Then click on Sessions.
In the SQL management tools, you can list the the connections to a database, but some applications will use a generic single connection so identifying the actual users depends on how your databases are configured.
For other applications, it's really up to the app.
You might be able to use Net Stat or other tools to identify IP addresses connected to the system, but that doesn't necessarily give you information on WHO is connected.