The Rebbe's guidance on business partnerships:
Enter a partnership carefully. Consider it well before entering, and consult trusted friends before deciding (IK 7670, 9077). Entering a partnership in an unfamiliar field, without one's own expertise or reliable oversight, is unwise (IK 9376). Sometimes a simpler arrangement, such as selling outright, is preferable to a partnership (IK 1631). Partnerships even for holy purposes, such as building Torah institutions, need care to protect everyone involved (IK 2772).
No partnership that involves chillul Shabbos. Such a partnership carries both spiritual and material risk; seek an honest livelihood instead (IK 997).
Clear agreements between the parties foster good relations and success (IK 3678). Business growth should also benefit others in the community (IK 7670).
See also: פרנסה – יושר והלכה בעסקים.