Group text for iPhone. I used GroupMe, but found it a bit annoying. It would send out messages to parents asking them to "join" GroupMe, which I did not like. Also if a parent did not respond to a group message, the app assumed they were not going to "participate in the conversation" and removed them from the group. It also sent a message to everyone in the group every time you added or removed someone from the group.
Group text is a simple, group texting app and it works well.