Usually, practices hire new physicians with an initial salary guarantee, e.g., $150K/year. Once you make 'partner' you keep more of your billing and may share the profits from the practice depending on your contract. You are also expected to pay a certain dollar amount to achieve partnership and share ownership of the practice.
It's good b/c you obviously make more money as a partner.
There are two basic ways to setup partnership:
1) You take a percentage of what you bill; for instance, if you bill $500,000/year and the overhead is 50%, then you take home $250,000.
or
2) The physicians in the group split the profits equally. This system eliminates the incentive for your seniors keeping all the private patients and scheduling the Medicare patients for you. The disadvantage of this system is that some physicians in the group may get 'lazy'.
There are derivations of the above, and the overhead costs will also vary.