Ask HN: How to mitigate the risks of open sourcing a project?

A friend and I created a platform for organising simple and fun forecast challenge with friends / colleagues for major sporting events (www.scoragora.com). We launched for the football worldcup last year and add a decent user base (7000+ users wordwide).

The purpose of the project was to create something cool, used by a lot of people, to learn how to craft this kind of website, to experiment trendy technologies (node, grunt, angular, ...), and eventually to make a bit of money out of it (at least cover the expenses small).

I posted a link on HN at that time (https://news.ycombinator.com/item?id=7873348), and during the worldcup we received some very useful feedback from dev users, and some offers to help. It would seem very natural for me to open source the project to ease the collaboration, and avoid spoiling all this good will.

However, I would like to keep the opportunity to eventually make some money out of it. And I fear that if I open source the whole github repository, I risk someone forking my repo and making a business out of it (who said paranoid ?)... I guess that's a pretty common preoccupation when you want to open-source a project.

Now the rugby worldcup is coming in a few days and we have opened the platform again, so we will probably receive a handful of help offers (hopefully). What would you do ?