Ensuring replies to group messages go to the whole group rather than just the individual who sent the original When I send a group text - I get back replies as individual messages all in their individual conversations and no one else can see the reply - this is fine if I want them private.

But I have once seen where each group member was replying and we all could see the messages like a conversation string - how do I set it so when each group member replies we all see their answer and also it is on the same stream
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.