Sticky situation...

[33girl]

MYOB.

If this is something that's against their policies, the onus is on THEM to find out the information.

Sure it's shady and sleazy - especially if the sorority they're joining is a sorority that runs like yours. It's particularly shady & sleazy to do it on the same campus. But signing a sign-up sheet isn't joining. Or they might not care if their members have been/are in other sororities. Many service sororities permit double membership.