I help out a local organization with their small computing needs. I
am definitely not a professional admin but I can usually figure things
out for small-time things (and Linux is my forte otherwise).

Over the last 4 years, I've been running NT 4.0 Server to handle their
file sharing needs. They have a 10-user license. It is configured as
a Primary Domain Controller and we have about 8 machines connect using
Windows 98 and XP to map a shared drive.

They would like to add about 4 new users. What does this mean for the
"10-user license"? Does it mean 10 simulataneous connections, 10
logged in users, or what?! Does NT4 enforce this somehow?

If I need to upgrade the OS (keep in mind that their needs are simple
and NT 4.0 Server as served us well), I've heard that domain
controllers no longer exist. Is Windows 2000 Server the way to go and
will going from a primary domain controller to the new thing be hard
to do without spending loads of time reading?

Thanks and sorry for the cross-post but I'm looking for an answer