Unfortunately you have to buy an app for this functionality as its not something you can do on the iPhone.
This is one feature I really miss from my Nokia days
There is a workaround from memory where you have to use notepad copying and pasting numbers and then saving as one contact.
A bit too much work for my liking so purchased an app to do it.
Perhaps I am missing what the OP is trying to do, but why can't you go to settings>messages>turn on group messaging. The only time it'll be a pain is the first time you have to select all of the numbers in that group. As long as you never delete it you can go back to that saved text in messages and resend to the whole group at any time.