I do have a workaround for you to text multiple people. I need to do this also, and currently the SMS feature only allows one at a time, so you need to find out what the person you are trying to text uses for their cell provider, then a little research to find the carriers email. Example, AT&T still works with the old Cingular email, so if your friends mobile number is 111-222-3333 you would want to add an email address into their address card with the following...
1112223333@mms.mycingular.com
Sending an email to that address will send it as a text message. Since you CAN email multiple people from the phone, this is how I am currently doing it. Also, doing it this way, you can attach photos to the email, and send it to them via text as well.
Hope this helps a little!
Here's a couple of popular email addresses...
T-Mobile: phonenumber@tmomail.net
Virgin Mobile: phonenumber@vmobl.com
Cingular: phonenumber@mms.mycingular.com
Cingular: phonenumber@cingularme.com
Sprint: phonenumber@pm.sprint.com
Sprint: phonenumber@messaging.sprintpcs.com
Verizon: phonenumber@vtext.com
Nextel: phonenumber@messaging.nextel.com
US Celluar: phonenumber@uscc.textmsg.com
where phonenumber = your 10 digit phone number
Here's also a helpful link to more addresses:
http://www.notepage.net/smtp.htm