Useful legal definition of collective from that site.

From California Attorney General's Guidelines for the Security and Non-Diversion of Marijuana grown for medical use:

"A collective should be an organization that merely facilitates the collaborative efforts of patient and caregiver members – including the allocation of costs and revenues.

As such, a collective is not a statutory entity, but as a practical matter it might have to organize as some form of business to carry out its activities.

The collective should not purchase marijuana from, or sell to, non-members; instead, it should only provide a means for facilitating or coordinating transactions between members."