I have an Atrix with Froyo and have been trying to figure this out as well. I have been able to create my group, but couldn't figure out how to call up the group to send text or e-mails. Thanks to this thread, I revisited this issue and solved the problem or figured it out. First, I am using the stock contact app, using Handcent SMS and Gmail.
To create a group select the contact icon> menu> display group> create new group. (You will see other groups you create, in this popup window as well, to edit or delete.) Name your group, then select the add button. From there, select the names and select done.
Note: You will only see your groups under the "display group" button. (Haven't figured out how to create a short cut for the home screen.)
For texting in Handcent, using one of the groups you created, launch Handcent> create new message> select the "+" symbol in the to field> group tab> then your group to text. If your contact has more then one phone number, you will be prompt to select the correct one> select end and compose your message.
For e-mail using Gmail, launch Gmail app> select compose new email> select the "+" in the "to" field> select the "down arrow" at the top of the screen or menu> display group and select your group. You will see your list of contacts and have to select each one in the list to include, in the email.
I hope this was helpful to you all.