This article describes how to send a group message.
INSTRUCTIONS & INFO
Send messages to a group (iMessage and MMS):
Tap the compose icon.
Enter multiple recipients.
Group messaging settings:
From the Home screen, Go to Settings. Note: With MMS, group messaging must also be turned on in Settings > Messages, and replies are sent only to you, they aren't copied to the other people in the group.
Tap Messages.
Tap the Group Messaging toggle to turn on or off.
Leave a group message: If the group message is using iMessage and has more than three participants, you can leave the conversation. Tap Details.
Tap Leave this Conversation. Note: Leave this Conversation might not be available if there are only three people in the group iMessage or if the group message uses MMS.