"It's a lot to think about, but once you build it, [proving MRBs with services] won't take a lot of effort to maintain," Crawford said.

Moreover, financial regulators don't necessarily want credit unions and banks to ignore MRBs operating legally under various state laws, Crawford said. However, regulators do expect adequate risk management, keen observation and adherence to current laws much like they would for other higher-risk clients.

Serving MRBs also depends on understanding the industry, Crawford said. In addition to growers, processors and sellers, related providers include the landlord who rents the space to the shop or medical dispensary, vendors who provide lighting, retail and security equipment, and even suppliers of basic business services, including accountants and lawyers required to make any business a success. Credit unions and banks that provide necessary financial services are also part of that service equation.

Under the federal Controlled Substance Act, marijuana is still considered a Schedule 1 drug along with heroin and LSD, and the Financial Crimes Enforcement Network offers restrictions to doing business with MRBs operating outside of legal state statutes.

The emergence of legally operating MRBs prompted U.S. Department of Justice Deputy Attorney General James M. Cole on Aug. 29, 2013, to issue the Memorandum for All United States Attorneys: Guidance Regarding Marijuana Enforcement. The "Cole memo," as it has come to be called, provided guidance to federal prosecutors concerning marijuana law enforcement in all U.S. states and serves as guidelines for financial institutions that serve MRBs, Crawford said.

The Cole memo's restrictions operate along much the same lines as those governing alcohol sales. Marijuana and marijuana-infused products cannot be sold to minors or be used as a pretext for covering up illegal activities. Profits cannot be channeled to criminal enterprises, and the use of firearms in any aspect of marijuana activity is forbidden.

In addition, marijuana cannot be grown on federal lands nor used on federal property. "Drugged driving" is strictly prohibited, as is distribution of marijuana in states where it is not legally allowed to be sold, something that Crawford believes would be difficult to enforce.

"How can you prevent products from state-authorized MRB activity from crossing into other states?" Crawford asked. "Are we going to set up a checkpoint at the Nebraska border and inspect cars coming and going?"

FinCEN provides guidance for financial institutions that want to serve legally operating MRBs, but leaves the final decision up to the institution pending directives and restrictions outlined in the Cole memo. An institution's capacity for risk and its due diligence to business practices are critical to serving this group effectively, Crawford said.

Care should be taken to make sure the MRB is operating under guidance of the Cole memo, as well as within the laws outlined by the state in which they're domiciled, Crawford said. The consultant suggested creating broad policies as well as detailed procedures in serving this group to forestall any unnecessary board involvement in daily operations.

Financial institutions with concerns about their MRB clients have the option of issuing "Marijuana Priority" suspicious activity reports to authorities if any red flags appear with any member account. Concerns may focus on MRBs that earn substantially more revenue than expected, fail to adequately account for that revenue, or make significantly larger cash deposits that might be expected from the amount of business being done.