I can verify Bertha's response about her school's parents being made well aware they aren't wanted at the "other" (now technically "their") middle school's PTA--I'm a parent at the "Other" school, and to make Bertha feel better, they don't even want their OWN parents there, let alone hers!
I too, have become a a rather pesky noisome mold at the school--even if I'm not allowed to particpate in things (our school group is notorious for letting people sign up for committees, then never calling them to let them partake in the fun--they only choose their friends), at least I can take a small amount of pleasure in being a thorn in their sides
! Seriously though, any parent wanting to be in the parent group should be THANKED and WELCOMED with open arms--not dismissed and ignored. Ask at the main office who the PTO pres is, and try contacting that person. Hopefully it's just ignorance on their part.
Try not to take it personally that they don't want you--it could be they're feeling defensive because you might actually know what you're talking about, and they're afraid you're going to "show them up." Try making a meeting with the principal about this, and offer to be the membership chair if they don't have one, or PR person if you really want to get under thier skin!