Open Messages
Tap the Compose button
Enter the names, phone numbers, or email addresses of the people you want to include
Tap the group name or contact suggestions as they appear
Type your message
Tap Send
To reply in the same group, open the group conversation and type your message
To start a new group with existing contacts, create a new message and add multiple recipients
If you want everyone to be able to reply to the group, make sure group messaging is enabled in Settings
If needed, turn on iMessage for group chats with other iPhone users in Settings > Messages