GUI for managing NIS
No, not one I've ever seen. You typically manage it via text files, these files are often under RCS, CVS, or SVN control. I usually keep the source files in this directory:
This is where I check them out to work on them, etc. When I want to propagate changes I
cd to this directory:
Here's where I maintain a Makefile which I use to rebuild NIS maps.
Example lines from Makefile
YPSRCDIR = /etc/yp
YPPWDDIR = /etc/yp
GROUP = $(YPPWDDIR)/group
PASSWD = $(YPPWDDIR)/passwd
SHADOW = $(YPPWDDIR)/shadow
GSHADOW = $(YPPWDDIR)/gshadow
ADJUNCT = $(YPPWDDIR)/passwd.adjunct
If you're really dissatisfied with NIS, and you want a more modern approach that includes GUIs, I'd suggest looking to LDAP. NIS is a essentially a dead technology, wrought with security problems. It's still useful in certain situations but LDAP is the future.
Determining logged in users through NIS?
No this is not possible with NIS. The NIS technology is purely to disseminate information from a central location to a pool of systems. It's one way. The information only flows from the central NIS server to the clients, the server has no visibility to any of the goings on on any of the NIS clients.
Again LDAP would likely be a better technology for these types of features. To find out what users are logged on within an NIS domain, you'll need to put together a script that shows you who's actively logged on on each of the 100+ machines you're using within the NIS domain.