If your group is a 501c3 tax-exempt organization, you can ask the IRS for a copy of your original filing (form 1023). That would have a copy of the bylaws that existed at that time. Some states require bylaws filings also. Check with the nonprofit or corporations division, usually in the secretary of state's office, to see whether you might be able to obtain a copy.
You're certainly not the only group to have lost the bylaws -- to a certain extent it's an consequence of the frequent turnover in parent groups. And the situation you mentioned, where people "remember" the bylaws as suits their situation, is also pretty typical. The alternative is to create a bylaws committee and write some new ones that fit your group's current operation. In fact, it sounds like you should do that anyway. We have some very good examples of PTO bylaws in the File Exchange that you can use as a starting point. Good luck!
- Craig
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very good examples of PTO bylaws: https:/classic.ptotoday.com//filesharing/category/62-bylawspolicies