Thanks for contacting TextNow Support!
The way our group chats work is they create a new TextNow number, as you've described, and then adds each member to that central number, so that all messages that go to that number go to all of the members.
At this time, it's not possible to create a group that uses your own TextNow number.
I apologize for the inconvenience. Our engineers are working on a better solution for group chatting, but I don't have a target date for when that feature will be revamped.