There are state and federal regulations regarding the type of service you want to provide. Don't even try to open a company without completely understanding the business and having extensive legal help.
Those type of companies have very strict guidelines and it is harder to adapt an existing company than trying to open another.
It's hard to believe that in 2013, when you can do anything online, nobody came up with such a simple service of handing payments and paying taxes for others.
You can never have an account where others can keep their money, once you receive money in, it should be yours. So, timing is factor a here.
But again, a 30 minutes talk with an advisor is not very expensive. There are offices specialized in opening such firms.