When more than one person or group works on something together, they do it jointly. A jointly researched and written term paper should have both the authors' names on the first page.
If you and your best friend buy a car together, you do it jointly, and if your whole family chips in to fund your cousin's trip to India, you contribute jointly. You might hear that your favorite radio program is jointly supported by a few local businesses, or read in a poetry magazine that it's jointly edited by two people. Earlier, the preferred word was joinly, though jointly comes from joint and its sense of connection.