There is mondo-huge wiki devoted to Trek info at
Memory Alpha that can be used to find places that your Marines might have served. Given the current timeframe of the game, your most recent postings/assignments would probably have been related to the Second Federation-Klingon War (2372-2373) and most recently, early engagements in the start of the Dominion War (Memory Alpha has detailed info on both of these conflicts, of course). Prior to that, look at possible training deployments, posts aboard starships, bases, etc... Maybe your unit has even made excursions into the Gamma Quadrant before (hence your particular platoon being assigned to this mission to begin with)... The Wormhole to the Gamma quadrant was 'discovered' in 2369 (shortly after the Cardassian Occupation of Bajor ended and the Federation took control of DS9) and there was some initial exploration of the Gamma Quadrant in the following year or so before the first encounter with the Dominion and their Jem'hadar soldiers.
As far as coming aboard goes... Cerberus is still in spacedock at Starbase 10 and will be there for the next 3 days (game time), pleanty of time left for the MACO unit to board before we ship out to DS9. My recommendation, honestly, since we've picked up a CO for the MACO group, is to coordinate with those players and determine where/when the Marines will board (be it at SB 10, en route to DS9, or after Cerberus arrives at DS9 - all valid possibilities)... I imagine that all non-comms (i.e. enlisted personnel) would arrive as a group (unless, just prior to this assignment they were on leave/liberty and received orders to report). The officers may arrive ahead of, along with, or after the platoon, of course... here's the MACO contingent as it stands right now:
Platoon CO - Capt Jasper Caide - Vesper
Platoon Command NCO - MSgt Brianna McQueen - Brianna
Alpha Squad Leader/Fireteam Alpha 1 Team leader - SSgt James Taggart - Suicidolt
Beta Squad Leader/Fireteam Beta 1 Team Leader - GySgt Rhiannon McQueen - Dragon Mistress
Chatter amongst yourselves is encouraged where the MACO detachment arrival is concerned... you guys are the unit, after all, and, as such, likely travel/live/work together.