That's not quite how it works. You can't have a group chat that's mixed iMessage and SMS/MMS.
If an iMessage user creates a group chat where not everyone is using iMessage, then it's MMS. I suppose now it could be RCS if everyone's using a device and carrier that supports RCS, but I haven't kept up with that. MMS has a bunch of limitations relative to any modern internet messaging app, so people don't want to use that.
Some people are also very reluctant to install third-party messaging apps.