Well, in version 9, the postpone option is actually built into the product, though I personally wouldn't recommend looking at upgrading just yet until at least one more point release is issued. There are still a number of bugs to squash.
So you know, in Casper Suite 9.x when you set a policy to postpone, it uses jamfHelper with a hud style window and a drop down menu with postpone time choices. What I don't like about how it works is you must assign a drop dead date when the policy will run and not allow any further postpones. This has the effect that if someone is away for several days and comes back to their Mac, they may have already hit the drop dead date and not have the option of postponing. Not very user friendly if that was the goal.
If you'd rather have a method where users can postpone up to X number of times before becoming mandatory, regardless of when they first see the message, then look around here on the forums for some threads on custom scripts that achieve that.
As for checking if a machine is a member of a certain JSS group, yes, it can be done, but why would you need to do that? If you're talking about Smart Groups. you can simply assign the script with the "postpone" message to the Smart Group itself. No need to check membership within the script, since the JSS handles that for you and would only run the policy on the machines in the group to begin with.
