Nevermind - I think I figured it out:
1) Create a group in the phone's native contacts app (ie. outside of the Handcent app)
2) Enter Handcent SMS, click on the upper right corner of the page where you can generate a new text
3) In the "To" field, over on the right is a plus (+) sign. Press it.
4) There should be a window that shows with 3 tabs at the top, "Contact", "Group", and "Recent". Touch Group.
5) Select the checkbox next to the Group you setup in step 1 above. (May have to scroll down to see the group.)
6) Back out of the window, taking you back to the Handcent compose text message view.
You should see the "To" field populated with the phone numbers of the recipients listed in the group you setup in step 1
This is how I got it to work; can't guarantee it'll work for everyone..... :-/