There are some engineers I know that would be good to have involved in the bea but did not register at Area51. Is there a way to get them private beta accounts?
This is a fantastic idea, especially for sites that move into a second week of private beta. If a site generates enough questions and answers in the first week, then all's well and it opens to the public. However, when there is not enough content being generated it makes sense to extend the user base.
I was mulling over this trying to come up with ideas such as having notifications on Area51 or collecting more commitments than necessary and rolling out invites, etc. All of them were crap and you hit the nail with a plain and simple solution. Simply let the existing private beta users hand out invites!
This could possibly be taken a step further and made such that if it goes into a second week, then you get one additional invite for every 50-100 rep (not gained from association bonus). While the details will be worked upon by the SE team, I fully support this idea. I would suggest that you also bring this up at http://meta.stackoverflow.com which is where features requests that could benefit multiple sites in the network get discussed.
There is some thought of possibly allowing this kind of functionality. It is not a definite plan yet, mostly because the majority of private betas only last 7 days - the need for invitations in that short of a period is fairly sparse. It's still under some level of consideration.
However, the point is rather moot, as this site is now open as a public beta. Invitations are no longer necessary as anyone can now join.