Workaround:

Suppose you have a "Cycling" SMS group
Edit all your contacts to have "Cycling" as a nickname
Write an SMS to "Cycling" (actually write the word cycling in the white box)
Press send
Phone shows a list of people with that nickname (it shows their full name)
Just tick all the people it shows

i.e. phone remembers all 15 -20 for you, only way to miss one is not tick it. To send messages to only certain group members, tick only them

To edit contact:
Open Phone
Scroll to contact
Tap into contact
Menubar>Edit
Add Field
Scroll to bottom of field types
Choose nickname
Enter "cycling"