You can't if one of the group participants does not have iMessage; then all of the messages must use SMS and MMS. MMS always uses cellular data, because it is a carrier service, and your carrier does not know about your Wi-Fi connection. If you leave cellular data on, and turn on use SMS in Settings/Messages it should work.
You can't if one of the group participants does not have iMessage; then all of the messages must use SMS and MMS. MMS always uses cellular data, because it is a carrier service, and your carrier does not know about your Wi-Fi connection. If you leave cellular data on, and turn on use SMS in Settings/Messages it should work.
You can't text someone without an iPhone using Wi-Fi.
Text (SMS/MMS) do not use Wi-Fi.
Why do I have to use my data to text someone who doesn’t use iMessage?
