A very big problem, indeed. I assume since the OP has been able to convince others to join them in the first place that the idea and what they're working on is

A very big problem, indeed. I assume since the OP has been able to convince others to join them in the first place that the idea and what they're working on is compelling enough. Therefore if they're leaving so quickly there is most likely something wrong with how they're managing. That is extremely high turnover. My approach to leading isn't like what I see most others doing, it takes time and a lot of effort but I find it to be well worth it in the end. A lot of it has to do with being personable and understanding who the people you're leading really are and what they care about. You have to be genuinely interested in them, though, otherwise it would just come off as kind of creepy. With that in place it allows you to talk, and that's really all there is to it. Talking.