Off-hand, I can envision one (probably old-schoolish)
approach:
Hold the user's online/offline status in the database table
you use to manage their accounts.
When a user logs in, tick their record in the database to
indicate they are online (isOnline=1).
When the user logs out, change the value back to "off"
(isOnline=0). You could also (depending on what version of CFMX you
have the approach would be different) set this value to change
automatically when the session ends/times out (see OnSessionEnd --
http://livedocs.adobe.com/coldfusion/7/htmldocs/wwhelp/wwhimpl/common/html/wwhelp.htm?context=ColdFu...
To display who's online, you could employ some fairly basic
Ajax code to call a CFM template, query who's on and return the
data to the Ajax script, which would output the data to the screen
(too much code to type here for an example, but check out:
http://www.w3schools.com/ajax/default.asp
for some basic tutorials (apologies if this is below your
skills...not sure what you may/may not know about Ajax but wanted
to include it just in case). Also, check out ajaxCFC for
implementing Ajax in CF apps faster (
http://ajaxcfc.riaforge.org/).
One other thing I'd be interested in hearing about (if it
applies) is how event gateways in CFMX 7 might be able to help
here. Event gateways are on my list of things to learn still and
might not apply here but I'd be interested to know if they did.
Anyway, hope this helps get you started...
Cheers.