If the practice is not forbidden by your campus, which would be defined in your Panhellenic by-laws, then you can certainly invite other chapters to participate. As 33girl said, it's important not to mislead PNM about the size of your chapter, but inviting other chapters has other benefits. It shows PNMs the benefits of national or international organizations, and the support units that exist in such organizations. At my campus, where we have a lot of locals, this was a selling point for the national groups. Mind you, the locals do just fine

Small chapters have to find something to focus on besides numbers...talk to PNMs about all of the leadership opportunities that are available to them joining a small chapter. Talk about the advantages of helping to mold the future of an up and coming sorority. You can't focus on the negative...it will show through in your presentations and drive women away.
Best of luck to you!!