If you e-filed your tax return there is no written signature on the form. You signed it electronically when you e-filed. If someone insists on having a signed copy you can print your tax return, sign it on page 2 of Form 1040, and give them the signed copy.
Form 8879 is not used when you file your tax return yourself with TurboTax. Form 8879 is used only to authorize a paid tax preparer to e-file your tax return for you. You don't have to authorize yourself to file your own tax return. If someone is asking you for Form 8879, either they think you paid someone to do your tax return, or they don't understand how tax returns are filed.
It sounds like you are dealing with someone who doesn't know much about tax returns. They may be working off of an outdated checklist with little or no understanding of what they are asking for. You might want to ask to talk to someone, perhaps higher up, who is more knowledgeable about tax returns. Or, if possible, consider going to a different organization.