In the stock app you start typing the first persons name, then select from your list, it then puts in that name and a comma if you tap the screen after the comma and begin typing again you just add more names as you want. A little long winded in my opinion.,
I use two different methods... the first being Handcent Sms app from the market which gives you all your contacts in a list with a tick box next to them, so tick the ones you want and away you go. A word of advice, in your contacts/phonebook you need to have their mobile number as the first in the list otherwise it doesn't 'see' it on quick select.
The other app I use is MeContacts which gives you a page of tiles and each tile is associated to a contact. You can name the group of people you put on a page eg friends, work, family etc. You can then set it up to group sms from their. The only downside for this app might be small screen size on mini, but hey its a freebie app you could always try it!
