Open the Messages app
Tap the New Message icon
Enter multiple contacts in the To field
Type your message
Tap the Send button
To create a group chat, make sure all recipients use iMessage or have group messaging enabled
To send to an existing group, open the group conversation and type your message
To add someone to a group iMessage, open the group chat, tap the group name or icons, then tap Add Contact
To remove someone from a group iMessage, open the group chat, tap the group name or icons, then tap Remove Contact
To name a group iMessage, open the group chat, tap the group name or icons, then tap Change Name and Photo
To send a group text to non-iPhone users, enable MMS and Group Messaging in Settings
Go to Settings
Tap Messages
Turn on Group Messaging and MMS Messaging