There are all different degrees of "unrecognized"...from the school doesn't care if they're there, they just don't support them financially, to "underground" groups that everyone knows are there but the administration doesn't admit it, to the silliness of some of the Northeast schools where you can be expelled for being a member.
Here's ddalne's other post.
There is only one chapter that is underground that we have to deal with. In terms of mixing with sororities, that chapter is slowly losing it's activities/associations with sororities. The things that concern me are
a.) they do not have to abide by "dry" drush rules
b.)they do not have to follow any of the rushing date restrictions imposed by a formal rush.
As far as a, if someone only joins a group because they're the only ones who have wet rush, they will fold quickly.
And b....I know you have probably heard this a million times before, but rush really is 24/7/365. Even if you cannot give out bids in between formal rush times, if you build a relationship with someone and let him know you would like him in the next pledge class, he's not going to turn around and join some other group just because they will give him a bid that day. If he would, he's a dork.
By the way ddlane, are these national or local groups or a mixture?