There is a strong historic precedent for what you have outlined. Below is a link to get you started to learn how the militias of East Tennessee and South west Virginia, formed to fight Indians, turned the tide in the Revolutionary War
http://en.wikipedia.org/wiki/Overmountain_Victory_National_Historic_Trail
They fail to setup viable structures that allow them to function similar to a 501C. President, Vice pres., Treasurer, Secretary, Sgt at arms, Legal and so forth. Set a code of conduct, background checks for all members seeking affiliation.
A few basic flaws that ultimately bring down groups, folks not being truthful regarding their past, folks not adhering to structure/code of conduct of the group (lone rangers), groups acting willy nilly w/o researching local, federal, state laws. Lotsa folks start talking nonsense during meetings, stuff that borders on sedition/conspiracy.
This sort of thing should be strictly forbidden and strictly enforced by any group.
The whole concept of militia has been demonized by the media to give a negative image. Hell, call it...America's gun club or whatever.
Fact is, like minded folks have the right to gather, discuss, research, practice as long as it stays within the law.