Leopard now allows for the creation of groups in the Accounts preference pane. When creating a new account, you can now select the type, and one of those types is "Group."
Of note is the fact that Leopard, unlike Tiger, does not create a group specific to the user. That is, in Tiger, when the user "systemsboy" was created, a group called "systemsboy" that systemsboy was a member of was automatically created. Apple has done away with this in Leopard for some reason.
If you want to add a user to one of your newly created groups, simply select the group and then add the user by checking his name. Multiple users can, of course, be added to a group.
Also of note, control-clicking the user's or group's account and selecting "Advanced Options..." from the pop-up will reveal additional account options formerly configurable only from within the now-defunct NetInfo Manager.
Accounts Preferences: Advanced Options
(click image for larger view)
In any case, now you know. And knowing is half the battle.