I can’t send group messages with iMessage

Hello masoudhy87, 

Welcome to the Apple Support Communities. 

The following Apple Support article can help you get started with group messages using your iPhone: Send a group text message on your iPhone or iPad. 

If you have never used this feature before, please contact your carrier to confirm that your current plan is supported:

To send a group MMS message on an iPhone, make sure MMS Messaging is turned on. Go to Settings > Messages and turn on MMS Messaging. If you don’t see an option to turn on MMS Messaging or Group Messaging, contact your carrier to see if your plan supports Group MMS Messaging.

Please reply back if you need more help with group messages.

Cheers!