I text to groups of up to 15 people, no problem. It's how I rally the troops when we're about to go out snow plowing. they're all sent to phone@carriermobile addresses.
Also, one issue that I've noticed in the last couple of months when doing this:
When a text message is sent from ANY rim device to ANY nextel device
(using the firstname.lastname@example.org address)
the text message comes across, but in front of it is a string of about 30 characters of nonsense. It's the exact same every time. It cuts into the capacity and people that don't frequent receive text messages may inadvertently delete the message before scrolling down enough to see that it's a legit message. I've spent MANY hours on the phone with nextel. It appears they weren't aware of it and I'm not sure they care.