Hi Laura,
I have done a few deals with family and friends, and the Most important piece to this deal is ensuring that whichever direction you move in is clearly explained to all parties, written down on paper (preferrably with an attorney) and signed off on by all parties. This will help avoid many headaches down the road.
I would recommend breaking out the roles and responsibilities into percentages and then working from there. In my eyes the downpayment/closing cost/upfront money is worth 50% of a residential deal, and all of the other items that go into operating that property (i.e. Property management, project management, etc) make up the remaining 50%. For example if the downpayment is $100k and they put in $50k i would give 25% of the overall ownership of the deal and the cashflow to that individual. If you are self managing i would recommend taking a self management fee of 6 to 10% (as if you were paying property management) and then start splitting on the profit coming in.
There are many directions you can take this, the above are just high level ways that i have done it in the past