Find ways for low/mid level developers to see that this will help to advance their career. I've been a part of 1 open source project, and that was normally the question I would see before a developer would decide to chip in. They want to see that you the founder/CEO are invested and won't decide to drop off the project from a funding or marketing perspective. (a lot of) developers want to see their name on the top of the contrib. list at the end of the day when it is successful.

one very small example / aspect that I can contribute.