Open the Messages app
Tap the New Message icon
Enter multiple contacts in the To field
Type your message
Tap Send
To start a group chat with iMessage, make sure all recipients use iPhone and iMessage is enabled
To enable iMessage, go to Settings
Tap Messages
Turn on iMessage
To send group MMS texts, go to Settings
Tap Messages
Turn on MMS Messaging
Turn on Group Messaging
Make sure your carrier supports MMS and group texting
If needed, add contacts from your Contacts app before sending the message