Open the Messages app
Tap the new message icon
Enter multiple contacts in the To field
Type your message
Tap the send button
Make sure Group Messaging is enabled in Settings
Go to Settings
Tap Messages
Turn on Group Messaging
Use iMessage for group chats with Apple devices
Use MMS for group texts with non-Apple devices
Ensure MMS Messaging is enabled in Settings
Go to Settings
Tap Messages
Turn on MMS Messaging
Check that your carrier supports group SMS
Verify you have a working cellular connection or Wi-Fi for iMessage