Hello katiebelbee,
The resolution would actually be the opposite. You'd all want to get iPhones.
iPhones have access to Apple's exclusive iMessage service. iMessage goes through Apple's own servers which allows multimedia messages to be a much larger size. This equals, full resolution pictures and the ability to send large video files without compressing them.
When you add someone to a group message that is not using an iPhone, it forces the messages in the conversation to go through the normal SMS/MMS messaging services through your wireless carrier. Since MMS messages can only support a very limited size, pictures/videos sent through that service will be compressed and have a significant reduction in quality.
If that person doesn't want to upgrade to an iPhone, then there's a few other things you can do to get around it. You can remove them from the group message, or create a separate group message with everyone but them in it. Use that second message thread specifically for when you want to send pictures and videos. This will allow the pictures/videos to send at full, original quality since everyone in the group is using Apple's iMessage service. If you need to send a full resolution picture/video to this person, you'll want to send it another way like through email since an Android device only has SMS/MMS messaging and can't receive full resolution pictures that way.
Cheers!