As a 501(c)(3), if your group takes in (total for the year) more than $25,000, then you are required to file Form 990.
Another common question is what defines "takes in". If you guys sell $40,000 in gift wrap and profit $20,000 on the sale, that counts as $40,000 in gross income for your group. So, the majority of groups should be filing.
Doesn't mean that you have to pay taxes on that revenue, just means that you have to file a report with IRS. We actually recommend that all groups file the report, even if you are below. It's a good habit and ensures that your group stays "live" with the IRS. Provides a nice paper trail, too.
One of our NPN Expert Guides (for NPN-member groups) is called "PTO Dollars and Sense" and it makes this stuff (and the forms) really simple.
NPN =
www.ptotoday.com/npn
Tim