A multiple member LLC as well as multiple sole proprietors working as one in the pursuit of profits, make up a partnership.

In most states a general partnership does not need to be registered at the Secretary of State. 

At year's end, the company files a separate tax return from the owners, but owner's still claim their share of profits or losses on their personal tax returns. Partners can be individuals or multiple companies.  This structure is ideal for temporary projects. 

Partnership profits are typically not subject to self employment tax.  However, absent and even despite a written agreement, under a partnership, you may be responsible for the acts and promises of your partner.  Your personal assets can be at stake in litigation.