The beta groups are primarily peer support groups--you'll see MVP's and
other peer folks answering questions as best they can.
Microsoft staff involved in the development of IE8 are reading the group,
however, and occasionally they will post to clarify an issue if they believe
it highlights a bug or something that can be fixed in the product.
Beta1 was oriented towards the large community of web developers--technical
folks who know one end of the browser from the other (which sure doesn't
describe me!) and will be interested in how IE8 handles their code--i.e. the
web sites and pages they have developed and are developing.
Beta2 is sufficiently robust and close to a final product that it is being
publicized for end-users. At this point, I suspect we'll see a large
increase in post volume, and perhaps a decrease in Microsoft presence---they
just can't respond to the volume of messages--even those that might seem to
call for an "insider" response.
So--building a community of peers who know just a bit more than the guy
asking the question and are willing to spend some time reading and
responding is important to the health of the community--i.e. the perception
of whether the group is a good place to take your question--whether you get
an answer, and reasonably quickly, and without being flamed for your
inexperience or lack of proper technical background.
As with most "general" groups, I would expect that group to be pretty wild
and wooly for a while, at least--everybody will be posting their favorite
feature in browser x,y, and z, and wondering why Microsoft could have been
so stupid as to have not implemented it, etc.