An easy solution is to have uneven ownership. If you're quitting your day job to work full-time on something, and I'm drawing a salary and working 20 hours/week on a side project, I'll expect you to be a minority partner.

Vest everyone's shares too - if I bail on you early, I might just have a few % of the company when I leave.

I think it's OK to have varying levels of commitment. Of course, the CEO should be 100% in the game. But I've worked with developers part-time, as partners, and found them reliable and committed with a lower number of hours.