J,
But that only works for the first message. If the OP responds, then if
Mark wants to respond the email will come from his own address. I
really hate the way this works.
So don't do that.
That is, if the OP responds and further communication is required, always send it from the (pending) members list - don't simply reply in your email.
Or, use your email service to "spoof" the group's +owner address. I've done that with some of my Y!groups. Y!Mail, Gmail, and most other services have a way to do that (as an account set up) so long as you can verify the spoofed address. The spoof isn't as secure as using the actual +owner address from the web site, but it is often a convenient compromise.
Shal