When we did our beta invites, we had about 30% signup. Of course, we were working with a small sample (about 100 invites), all of whom were personal / culled contacts. We used the "beta" label in the old fashioned sense, which I guess is now more appropriate to call an alpha.
I would suggest that rather than comparing to other groups, you really focus on that as your baseline metric. Look at it as a time-series, not inter-company. So you have 7% and 15%; what can you do to raise the 7% to 15? Realize that it'll always be more difficult to get people to return to signup, versus when you have their attention on your page, so I would expect on-page signups to be higher.
What's your product? Just curious.