We've never accepted money for fuel and never will. We'd be out on the boat regardless of who else is in it. If someone wants to pay for lunch, fine. Generally speaking, guests are going to be doing things my way, which includes where we go, where we dock/anchor, and pretty much their entire journey.
If we're out with other family or close friends, we tell them ahead of time what we're planning to do, whether it be anchoring/swimming or heading out for food at a restaurant. I want them to be prepared, so bring what you want for food/drink and some towels. It doesn't cost me anything extra (usually) to have another couple or a few extra people aboard.
Now it's a little different for those that have friends that meet up with your for watersports in pleasure boats. Same with fishing trips.