In the past, we have discussed the admisisons myth, the closed waitlist is a test, and have subsequently advised candidates to trust admissions officers at their respective words, meaning avoiding communicating with those who have expressly stated that they want no more information from you. So, that written, candidates should also avoid the opposite extreme: if a school says that it is open to important communication from you, then you should not interpret this to mean constant communication. There is, of course, a difference.
As is the case with any waitlist situation, before you act, you should carefully read the waitlist letter sent to you by the admissions office - frequently, they will include an FAQ sheet as well. If the school permits additional information. but offers no guidance with respect to quantity, that does not mean that it is open season. If you have another potential recommender who can send a letter that highlights a new aspect of your profile, you should certainly send one in, but you should not attempt to start a lobbying campaign where countless alumni and colleagues are writing letters and emailing on your behalf. Similarly, you might send a monthly email, an email every six weeks or even an email every two months - the key is not frquency or volume but materiality. If you have something important to tell the admissions committee that can help shape their perspective on your candidacy (a new project, a promotion, a new grade or GMAT score, a visit to their campus, for example) then you should inform them. If you do not, then a contrived letter with no real content will tell them nothing and will not help you. Just because you know others are sending letters, you should not feel compelled to send empty correspondences, for fear that others will be showing more interest. They just might be identifying themselves negatively via their waitlist process.
Let's take a step back: imagine that you are on the admissions committee and you have one candidate who keeps you up to date with a few thoughtful correspondences that truly count and another who bombards you with empty updates, emails and recommendations that don't offer anything substantive. Who would you choose if a place opened up in your class? Your goal when you are on the waitlist should be to remain in the good graces of the admissions committee. Remember, they already deem you to be a strong enough candidate to take a place in their class, so be patient and prudent and as the process progresses, as challenging as that may be.