There are many different business ventures where you can share capital and become an official partner of the company. This may not necessarily be a kind of company where you are required to purchase shares in order to invest in a business venture because there may not be hard and fast rules about how many people could become official partners.
I don't think that there's anything like like selling of shares in a partnership business. It's only in a public and private limited liability companies that shares of the company are sold to the general public who are interested in buying to invest in the company. It doesn't work that way with partnership business.