My chapter was in a similar situation... much smaller than the other sororities on campus. I don't think national HQ ever actually threatened to pull our charter, but they are "concerned" by our low numbers (especially the year that we came out of formal rush with only one new member!), and they have sent people (every year, I think) to help us with rush.
Here are some suggestions based on things we've done:
- During formal rush, PNM's will notice your small size and may cut your sorority for it. To cut down on the impact of the size difference, hold your parties in a smaller room, and fill it with balloons and decorations.
- During formal rush, get area alums to help out. If there's a nearby chapter, see if they can loan you a few sisters. If they are allowed to rush PNM's, great - just make sure the PNM's meet some actives from your chapter! If they're not allowed to talk to PNM's, have them distribute refreshments, etc. This frees up your actives, makes the chapter look bigger, and emphasizes the "national-ness" and lifetime commitment of being in a sorority.
- COB constantly. Get your sisters to bring their non-affiliated friends, roommates, etc. - even people they think wouldn't dream of going greek.
- If someone approaches you about joining and there isn't a rush week coming up, strike while the iron is hot. Have a bunch of sisters take her to dinner or something, and extend her a bid if you like her.
- Don't be afraid to take a new member class of one. They add up!
- Consider all women, including juniors and seniors.
- Be selective. As others have said, don't just hand out bids to anyone. Your national is probably going to be more impressed with 30-40 quality women than with 50-60 random people.
Whew, I wrote a novel

Hope it helps!