Comic fan clubs have been around almost as long as comics themselves. DC's "The Junior Justice Society” and the “Supermen of America” clubs dated back to the 1940s. For ten cents, a subscriber received a certificate, a decoder, a pin-up poster and other miscellaneous collectibles. For years Marvel resisted starting up a club out of fear that many would compare them to jumping on the bandwagon when they were trying to be different. Around 1963, Marvel was in the middle of creating some of it's most creative characters and Stan Lee decided to finally start up a club for all the new readers. The idea Lee had was that he wanted to relate to his audience that Marvel wasn't some unfeeling mechanism of enterprise but a comrade, a member of a team in which you, the reader, could become part of.
For months, Stan and his "Bullpen" promoted the M.M.M.S. through its lineup of titles without revealing what the letters stood for. Fans wrote in, attempting to be first in cracking the code. Some suggestions were "Make Mine Marvel Stan" and "Marvel's Money Making Scheme". Finally, the club was made public and its membership quickly escalated.
MembershipFor a one dollar contribution (and 75 cents for returning members every year), each member would receive an official membership kit that included several items. A letter welcoming you to the organization, a giant-size membership button with the declaration "I Belong: The Merry Marvel Marching Society," a collection of stickers, a membership card proclaiming you’re a charter member, and a pledge of allegiance to Marvel. You also received a 33 1/3 rpm record with the voices of the Bullpen gang clowning around and welcoming you to the M.M.M.S. with a theme song. 
Each year a new kit would come out and members needed only to make a 75 cents donation for the latest items. Changes were made so that no kit looked the same and made it feel like the fan club was actually growing and making changes to define different members. The buttons went from "I Belong" theme to "Make Mine Marvel", and the membership cards changed colors every other year.
Additions & Changes
To living things up, more items were added to the club to get more members and keep current ones. The Merry Marvel Messenger was a newspaper item that featured bios of the Bullpen staff, news & gossip, trivia, and sneak previews for MMMS'ers only. Another item was Marvel Stickers which could be stuck to practically anything. In 1969 the club introduced Marvelmania Mad Money which couldn't be used to purchace anything and served no purpose other than to be collected.
Decline & Others
Entering the 1970's, Marvel began to have a decline of readership, which reflected on the club and it's membership renewal at the start of the year. Several offers were sent out for new members, but it was clear that there was not much of an interest in the club anymore. Marvel slowly started to fade the club away while starting new ones such as Marvelmania International and Friends Of Old Marvel. While these clubs had varied success, they didn't compare to that of the M.M.M.S. By early 1971, the original club was gone.
The character of Hiro Nakamura on NBC's Heroes is reported to be an M.M.M.S. member in the episode "Don't Look Back."