We will be traveling from Germany to USA and have 23 hour layover in Atlanta. I have an American Passport. The other travelers have German passports.
Having an American passport usually means you are an American citizen. As such, you can enjoy unrestricted travel in the US.
The other travelers will have to be admitted to US before the start of the 23hr layover. If they are admitted, they also enjoy unrestricted travel.
Remember, that there are no transit zones in US airports. Immigration checkpoint is passed (usually) at the airport of the first entry.
So, yes you and your companions can leave the airport.
If you are transferring to another city in the US via Atlanta, then yes. You are going to go through immigration in Atlanta, after which you will be landside, i.e. you have essentially left the airport. So there will be absolutely no problem for you to leave the airport and do whatever you want, regardless of whether you are an American citizen.
If you are transferring internationally, the same applies. In the US, all transfers, domestic or international, requires passengers to go landside, passing through immigration checks. So nothing changes.