FusionForge is not a huge project (Ohloh lists 13 contributors currently), so we don't have a very formal organization. If you recall the initial FusionForge announcement, one of the goals of the project was to have an explicit and transparent governance model. We have so far fallen short of that, and although we believe we are more transparent than we used to be, there's no formal decision-making process. So far (after a year of existence), we haven't been facing a decision point where consensus wasn't reached. The motto of the IETF ("We reject kings, presidents and voting; we believe in rough consensus and running code") could therefore be said to apply to FusionForge too.
For questions that go further than simple technical details, the decision system is currently to discuss them on the #FusionForge IRC channel (on the Freenode network) during the weekly meetings which are held every Friday at 1:30 PM, Western European time (TZ=Europe/Paris). The meetings are open.