Joint ownership in patents

Are you talking about being a joint owner or a joint inventor? To be a joint inventor, you must have contributed something to the invention, and not necessarily an equal percentage of the overall invention. As for being a joint owner, that is basically a contract question - for example, did you invest in the development of the invention, and therefore in return for your investment, you have a right to be a part owner of the patent if that was part of your original contract upon which your investment was made. Alternatively, did the inventor work for you, and therefore, per his employment, he is required to assign part or all of his rights to his invention to you.

As Steven Weinrieb already has said: ownership is a legal issue. Sometimes you become owner as of right, e.g. if you are the employer of an employee-inventor (if the law or the employment conmtract says the employee has to assign his - rights to - invention to you). You can also become joint owner if the Original owner sells part of the patent (or patent application) to you.