Hello CCaroline711,
Welcome to Apple Support Communities. We understand you have a question about group texting while using Wi-Fi Calling on your iPhone. We'll include our main guide about using Wi-Fi calling below. The answer to your specific question is that you'll want to check with your carrier. Some cellular providers don't support the feature for group texting over Wi-Fi only. You might still need to have Cellular Data enabled in order for the text message to go through.
This is more evident if you are part of a group text that includes devices that are not iPhones. One way to test is to attempt to text a single contact that doesn't use an iPhone (green bubble). Once you've confirmed with your cellular provider that this feature should work, reply back here.
Make calls using Wi-Fi on iPhone - Apple Support
Wireless carrier support and features for iPhone in the United States and Canada - Apple Support
Warm regards.