Billing / Invoicing will be implemented at some later point.
As a native feature.
I sincerely don't think that we will be implementing billing / invoicing before the release of Collabtive 1.0
We have many, many other things on our agenda that we need to implement before thinking of such (quite advanced) additions.
In the upcoming time we will be focusing on improving timetracking, reporting, exporting functionality in Collabtive.
Also we will implement budgeting into the timetracker, which will eventually lead to the development of a billing feature in the future
But not in the near future.
Of course, we would be very happy if someone of you steps up to implement some desired functionality.
Collabtive is open source after all, and we are keen on merging outside contributions in our mainline version.