You also may be registered with your state as a nonprofit but not with the IRS.
If you do not need to solicit tax-exempt contributions or receive grants from foundations, you don't really need to be a registered nonprofit.
Check with your state (usually the secretary of state's office) to find out your status. If you are incorporated as a nonprofit there, then check with the IRS to see if your organization is already registered.
If not, you could then apply to the IRS for 501(c)(3) status if you think it would be advantageous. Consult legal counsel and a CPA to see what would work best for you.