Ensuring replies to group messages go to the whole group rather than just the individual who sent the original
If you send a group message on your iPhone, anyone else who uses iMessage will respond to the entire group. However, if a message recipient is using standard SMS (such as a user on an Android, etc.), their response will be a standard SMS message back to you.

So if you want a true group message, ensure everyone is using a Mac or iOS device, with iCloud and iMessage enabled. If you are in a mixed environment with non-iOS users, you might look at a third-party messaging app that works on all platforms.