I wouldn’t stress. If people say there will be an extra guest I send them a change request. If they say there may be an extra guest but not sure I say they can pay cash when they get here.
The issue is having an “offline” unregistered guest. If there are issues like.damage or fall.down the stairs, they won’t exist to Airbnb and it is a violation of the TOS. There are issues with local laws too.
Amazon has a local store here and they don’t take cash! Having worked retail, it would be great no to worry about cash shortages, robbery, and a daily accounting and deposit. A host would complicate their paperwork and taxes taking cash, not that it would be recorded at.
Once a guest has made a cash offer via the Airbnb messaging system, the host pretty much needs to warn them in writing.
If you feel the need to do it that way then go ahead. A guest paying cash for an extra guest they haven’t booked but told me might come is not in my list of “100 biggest problems that keep me up at night”.