Partners only exist in the context of a partnership agreement.
There are two types of partners - general or limited. General partners may bind the partnership in regard to any customary and usual business purpose, and act as the agent of the partnership in this respect. The one defense is where a partner was not authorized to act, and the person with whom they dealt was aware of that. A general partnership agreement only needs to evidence an understanding between two or more persons. It does not have to be in writing.