Sorry if this sounds curt, but I'm a little frazzled these days....
First, here's a link to a decent article explaining the difference between being non-profit and being tax-exempt. It's geared more towards trade associations, but the same basic principles apply.
www.venable.com/publication.cfm?publicat...2&publication_ID=484
Non-profit vs. profit status refers to your status as a corporate entity. Tax-exempt refers to whether your corporate entity is liable for taxes at either the state or Federal level.
Here are the steps I would take--
1) Find out whether you are incorporated or not and what kind of a corporation you are. Ideally, you are incorporated as a nonprofit organization. Your state government offices should be able to help you figure this out.
2) If you are not incorporated, you are probably what is referred to as an unincorporated association, which is also most likely nonprofit.
It IS possible to be a nonprofit organization and NOT be tax-exempt. But that's not what you want--you want to be tax-exempt. So....
3) Once you figure out what kind of business entity you are, then you can file for your tax-exempt status. First, file for the Federal tax exemption with the IRS. I think someone gave you a link to the IRS that would be helpful, but you can also call the IRS and have them tell you what you need to file for exemption.
4) You may also need to file for tax exemption at the state level, but I'm not sure how NH works. I seem to recall you don't have state sales tax there, but I could be wrong. Your state corporations tax office should be able to tell you what you need to do. Sometimes, some states take your proof of Federal tax exemption and automatically make you exempt from having to pay state sales or income taxes. Some states make you go through a separate process to get exemption from income and/or sales taxes.
Net effect of all this....
In terms of your group paying taxes: you may not even have been big enough in the past to warrant filing a Federal income tax return anyway. The filing threshold is based on the total gross receipts of your group. If, however, you've been pulling in $50,000 a year in gross receipts (as an example), and you've never made a tax filing, you could get in some trouble (although my personal opinion is that it's unlikely). Some groups have gotten around this by just starting a new corporation and starting fresh with the state and the IRS.
You've probably been paying state sales taxes all along (if NH has them), so paying back taxes on that probably isn't an issue. Usually, companies require some sort of state paperwork before they will waive state sales taxes for you.
As for your accepting donations if you're not tax-exempt, that's probably not a big deal, either. It just means people can't legitimately deduct them as a charitable contribution on their tax return. Businesses have other ways of deducting their donations to you anyway, and most individuals probably don't think to itemize their contribution to you on their tax return. If individuals do deduct it as a charitable contribution, and they get audited, it could be an issue, but it would more likely be their issue to deal with.
Might want to clarify what people want when they tell you they want your "tax-exempt number." In my state, when someone asks for that, they are looking for the number on our sales tax exemption certificate so that they know we are exempt from paying sales taxes. Some people refer to your employer identification number (EIN) provided by the IRS as your tax-exempt number, but no one can look at that number and tell (just by looking at it) that you are tax-exempt through the IRS. However, they can call the IRS (or look it up online) and verify your tax-exempt status with that number.
It's worth joining the National PTO Network referred to above to go through this whole process. They have a very good step-by-step guide to help you through this whole thing.
Hope this helps!