In saying that you could post anything military related, you've made the group too generic, and it's lacking a focus. Tell me, how exactly would Star Wars fit in with a M1 Abram MBT or a Civil War vignette?
If you want people to actually be interested in the group for what it is rather than the hype generated by "plz join ma groop lolz", you need to start filtering content and setting some very firm guidelines.
For instance, since you called your group the USMC, why not just limit posts and discussions to merely the real-life United States Marine Corps? If you allow SW and random military creations in as well, your group's focus will be severely diluted.
I'm speaking from experience here - I have seen many, many KFOL founded groups go bust within weeks because they lacked content beyond the hype. I run a more focused group myself, and believe me, it's a lot of hard work doing the right thing, but it's worth it.
There. That's the best advice you'll be getting in a long time.