Also like a sole proprietorship, a general partnership doesn't protect against personal liability and in addition, partners are held responsible for the actions of the other partners.

WSJ: Small Business: Forming a Partnership

Self-employment activity generally includes ownership and operation of a sole proprietorship, Limited Liability Company (LLC), C Corporation, S Corporation, and Limited Partnership where the business intends to generate revenue for profit and make significant contributions to the plan.

FORBES: Think About Going Solo In Your 401(k)