You could create a shortcut with the recipients pre-listed. E.g.
Text message colleagues.lnk
32#"tmail.exe" -to "06xxxxxxx; 06xxxxxxx; 06xxxxxxx" -service ""
Will prompt the messaging app with a new message. If you last used Hotmail/whatever, though - it won't start a new text message, but a new e-mail/whatever.
If you want to be able to pick the account, use instead:
32#"tmail.exe" -to "06xxxxxxx; 06xxxxxxx; 06xxxxxxx"
If you want to add a default message, use :
32#"tmail.exe" -to "06xxxxxxx; 06xxxxxxx; 06xxxxxxx" -body "Text here"
You can also use vijay555's VJSMS to always send a text message, regardless of what account you're in.
32#"\Program Files\vijay555\VJSMS.exe" "06xxxxxxx; 06xxxxxxx; 06xxxxxxx" -body "Text here"
Essentially, all the above are more like 'templates' than they are group text message tools. If you have a new person you want to add to a 'group', you'll have to manually adjust the shortcut.
On the other hand - it's available now, and freely.