In order to send SMS/MMS to many other people, there are two ways.
1. Send SMS/MMS to group members as a whole
Long click group items.
Select 'Send SMS to Group'.
2. Send SMS/MMS to specific(any) members
Select the check box left side of contact name.
Click SMS button below side of Tab bar.
If more than one SMS program are setting up and processing, program selecting screen will be appeared.
Depending on the system, you can be limited to send SMS to others once.
In this case, I recommend using other third party applications like 'Handcent SMS'.