If you know their mobile carrier, you could just email the SMS to their phones. On AT&T it is phone#@txt.att.net but it is not really a group SMS as if anyone reply, it will only go back to your email and not to everyone in the group. But usually this is better for announcements to a big group. You don't want 50 people sending group SMS to say "Thanks!" "What time?" "Take me off this group I think you got the wrong number!" "Me too!" ... etc....