Well, this one is tough...
I've read, and even written, much about how parent groups need to involve the staff as much as possible, yet it might be a bad practice for a teacher to be placed into an officer role. Obviously by starting up your own group the staff would be the officers.
This is bad in general due to the fact that the Principal is your boss and as such can affect your decisions. A group of this nature needs to be free from being forced to do specific things, such as the things that a Principal may want.
For example, if my children's Principal wanted us to buy five new computers for every classroom, he can bring it up at a meeting, but we have the final say. Effectively we want his, and the other staff member's input, but we need to be able to make the decisions for ourselves without the influence that a boss (the Principal) might have over a teacher.
A second negative point is that you'll most definitely be competing with the existing parent group. That would go against everything that these types of groups try to do. Think about how the parents will feel when they hear that the teachers are starting their own group. They'll be wondering why you didn;t just join theirs.
IMHO, I would recommend getting more involved in the existing group. They would likely love to have more involvement by the teachers (we're complaining about that stuff on these boards all the time). Another route that would not be competing would be to get involved in your School Council (assuming you have one). Most schools do, and if yours does not then maybe that is a place to try to act (suggest it to your Principal). Usually there are state or town laws requiring School Committees and it dectates who is on it. For example, in my children's school there must be 4 teachers and an even amount of parents. The School Committee doesn;t run fundraisers, but can plan new programs and clubs and such. Some examples of the things that we have done this year as a School Committee are, Before School Taekwon Do, Math clubs, Reading clubs, science clubs, etc.
This would be a great way to go and if you need financial support the PTO might be interested in helping.
Good luck,
PresidentJim