It's a little unclear to me whether the issue includes multiple users on the same iPad, or is just multiple users sharing a common database. If it is the latter, yeah, you could just switch between different omni sync accounts (but you need to make sure you sync first!) though it would be a hassle if you had to go back and forth between the common database and the personal one frequently, and I'm not sure the notification system would work very well. If you ever forgot to sync after making some changes before switching databases, you would lose those changes.
You could do the same thing if sharing an iPad, but there would be even more switching around required, obviously, and your privacy would be only as good as your attention to the detail of switching back to the common database (or just erasing the current database on the iPad) when time comes to hand the device to someone else. Not my idea of a fun time!
A word of warning: OmniFocus wasn't built as groupware, and there are some assumptions made that only one device at a time is making changes. If two of you edit the same item without syncing between the changes, the earlier change will be lost, even if you edited different aspects of the item (you edited the task notes and the other person edited the due date, for example). Frequent syncs help lessen this liability, but best of all is if you aren't all trying to edit the same part of the database at once.