How do I assign multiple users to a particular case?
In principle our experience has shown that in real life, if a task or bug is owned by more than one person, it is owned by nobody and gets neglected. That is why FogBugz is written to assume one person per bug. (Of course, there are other "stakeholders" in the bug, such as the person who opened it, but there is only one person who owns it at any given time).
On the other hand, sometimes you have a team of people who are equally capabable of working on a set of tasks or bugs, and you want to be able to assign the bug to that team for a while before you decide who is going to work on it. There are two ways to do this.
Queuing cases for assignment
The traditional way is simply to assign it to the team lead. The team lead winds up with a bunch of bugs that they are not going to work on personally, but which they haven't yet passed out to the team members.
Another good way is to create an account in FogBugz for the whole team in addition to personal accounts. You can assign bugs to this virtual account, and tell the team members that when they have a chance, they should go through the bugs assigned to their team and then personally assign to themselves any bugs which they are ready to work on. For example, on a large project we once worked on, it took two or three people just to dole out the incoming bugs to individual team members. We created a virtual account called "New Bugs," with email notification turned off on that account, and set that as the project owner. The team leads were each responsible for checking the bugs assigned to "New Bugs" regularly, using a saved filter, and assigning the bugs to individuals.
If there are several people who need to be notified whenever something changes about a bug, they can all subscribe to the bug by clicking the small link at the bottom of the bug report. Users can opt-in to be subscribed to all cases in an area (or by other criteria) with the Autosubscribe plug-in to keep in the loop with everything going on.
See this post about subscribing other users to a case, or this post for requiring sign-off / review of a case by multiple users. For a one-time notification, you can assign the case to each user in sequence, or see this post for more discussion.