One way is to agree a virtual hourly rate for their initial input until the time that you raise funding. Then allow them to either purchase equity using their virtual dollars or if they want out, convert to real dollars from the funds you just raised.
If they want equity they convert their virtual dollars into equivalent equity based on your post funding valuation. Therefore, the more it’s worth the more money you raise and the less equity you give away.
If you also want to offer profit sharing then you can introduce a dividend payment to all shareholders based on a pre-determined percentage of profit.
If you don’t raise any money but start to make revenue then you can still apply your pre-allocated profit share percentage to the developers in proportion to their contribution/hours.
You will need to do your own projections to determine what a reasonable percentage of profit sharing would be in order to make it worthwhile since I have no information on your business model.
This way everyone knows exactly where they stand in each of these scenarios.