[qwhelan]

Goldman Sachs hasn't been a partnership since they IPO'd. They just use "partner" as a title for legacy reasons.

In general, a partnership is a collection of equity owners ("the partners"). To become a partner you have to buy equity, usually sold by a retiring partner.

Sometimes "new partners" are created by diluting the stock; this is generally avoided unless the person is expected to grow the business by at least that much.