The IRS can look up your EIN based on your group's name. They did this for me -- I had a number, but the IRS said their records showed that number had never been issued, so they looked up the correct number by the group name; apparently, someone had applied for the new number in 2003, but never passed the number on to the next board -- or even the bank!
I think the number is (877) 829-5500
You might also want to check Search for Charities
and make sure you have tax-exempt status. Having an EIN is *not* the same as being recognized by the IRS as tax-exempt.