What is the SFMC? Good question. Here’s a good answer from the SFMC website http://sfi-sfmc.org/
The STARFLEET Marine Corps is a group affiliated with STARFLEET, The International Star Trek Fan Association Inc. The SFMC is considered an office under the Vice Commander, STARFLEET and as such reports to Vice Commander, STARFLEET.
Fictionally, the SFMC is part of the Ground Forces that are rarely seen in the episodes but are assumed to be a part of the Naval Fleet of the United Federation of Planets.
We are members of STARFLEET. The Corps does not have different requirements for membership, except that a STARFLEET Marine must be a member in good standing in STARFLEET. Although it does have a different name for each rank within the promotional system, and a different organizational structure than the Naval portion of STARFLEET, STARFLEET Marines are regular, dues paying members.
Some STARFLEET members are Marine Reserve. These are members who are active in the naval portion of the Fleet AND with the Ground Forces area as well. Occasionally you may find a STARFLEET Marine who is or was active military, but this is not a requirement to be in the Corps.