I haven't seen much in the way of support of multiple user solutions yet.
The web app for OF could be used in a collaboration environment - but its all or nothing with a given copy of OF.
I've get a script in development that syncs selected data with a web todo service called Toodledo. Toodledo supports sharing specific projects with other Toodledo users.
So this script does allow:
OmniFocus <-> Toodledo(User1) <-> Toodledo(User2)
This means that user1 can have tasks visible, and or modifiable by another user with a fair amount of control over which projects are shared.
I'm hoping at some point this script will allow full OF - OF connections via
OmniFocus <-> Toodledo(User1) <-> Toodledo(User2) <-> OmniFocus
But right now the Toodledo API can't access the shared tasks
The project is at:
http://groups.google.com/group/oftd
-P