I have to disagree with my friends at the Cannabis Defense Coalition on this one. If you're asking what the law explicitly allows, it is not clear at all that 5 patients can grow together in one location - and even less clear that one person can be the primary gardener for a 5-patient garden . The problem you face is the legal concept of "constructive possession."
The Washington State Medical Use of Marijuana Act says that a qualifying patient, or his or her designated provider, can possess "no more marijuana than is necessary for the patient's personal, medical use, not exceeding the amount necessary for a sixty-day supply." RCW 69.51A.040(3)(b) . Possession can be actual or constructive.
"Actual possession exists where goods are in the personal custody of the person charged with possession. Constructive possession exists where a person not in actual possession still has dominion and control over the object or place whether the object was found . ... Dominion and control need not be exclusive and can be established by circumstantial evidence. ... To determine whether a defendant was in constructive possession of an object, we look to the totality of the circumstances. ... One aspect of dominion and control is that the defendant may reduce the object to actual possession immediately . ... While proximity alone is not sufficient to establish constructive possession, proximity coupled with other circumstances from which the trier of fact can infer dominion and control is sufficient to show constructive possession." State v. Chavez , 138 Wn. App. 29, 34-35, 156 P.3d 246 (2007) (citations omitted).
In the context of a collective or cooperative garden, it is likely that patients, their providers, the property owner, and certainly a designated gardener will be in constructive possession of more than one patient's plants at some point in time.
On January 13, 2010, the Seattle Police Department issued Directive 10-001, which accommodates two patient supplies in one location, acknowledging that a patient may also serve as a designated provider to one other patient. Under those circumstances, if identification and authorization for both patients is presented, SPD officers will leave 48 ounces of usable cannabis and up to 30 plants - double the presumptive 60-day supply definition adopted by the Department of Health and codified at WAC 246-75-010 .
Bottom line: yes, patients can grow together as a practical matter - and it's clear that some are doing this in Seattle. But I think it's questionable whether "patients have a [legally-defensible] right to grow pot together" as CDC suggests. CDC is right on the money, though, when they describe the considerations that should inform your decision whether to embark on this path.