Hi,
Its not currently possible but I will look into adding an option for that, I think it would be very easy to add it. I guess the reason its not there is that my initial conception of the blog was that it would a be a per user feature rather than something where multiple authors will be posting in the same blog. But I can definitely see that as a reasonable use case for some sites so I will implement it.
What you can do right now with mojoPortal is give each author their own blog then aggregate them together using the RSS Aggregator feature. If you do that, the RSS Aggregator doea show the author's name with each post already.
Hope it helps,
Joe