Question: Does anyone ever see the NPHC allowing inclusion of a 10th group?

[Rain Man]

To elaborate more on my original response to the OP's question (yes, there will probably be a 10th group in about 20 years, if not sooner), the biggest hindrance to the non-NPHC orgs is that their existence functions more as a regional GLO than as a national GLO. In other words, their exposure as far as how well known they are as an org is limited to the region they primarily exist in. For instance, here is a list of orgs and the primary region they serve:

Phi Eta Psi - Midwest, primarily in Michigan
Phi Delta Psi - Midwest
Phi Omicron Psi - Eastern seaboard (MD/DC area)
Phi Alpha Psi - Eastern seaboard (MD/DC area)
Beta Phi Pi - Midwest
Phi Rho Eta - Midwest, primarily in Illinois
Delta Psi Chi - Midwest, primarily in Wisconsin
Gamma Phi Eta - South, primarily in Georgia (assuming they still exist)
Xi Gamma Phi - South, primarily in Georgia (assuming they still exist)

Note that I am focused on those orgs that were founded prior to 2000.

The only non-NPHC org whose chapters covers a more broader area scope is Gamma Phi Delta. Unfortunately, I don't know to what magnitude, since it appears their website only lists currently active chapters and it looks like a lot of their undergraduate chapters are inactive.

Note that while G Phi D has chapters in multiple regions, there are no chapters in the Mid Atlantic (NY/NJ area), New England, and Rocky Mountain/Great Plains states. But I am sure that as time goes on, those areas will be covered as well.

Hope this helps.