I think you can let your friends use your vehicles all you want. They can even pay for the gas, that's only fair. They *might* be able to pay to offset some other operating expenses but that gets iffy pretty fast...
Where it becomes a problem is if you're getting any net positive revenue (or other compensation-in-kind)... trying to account for other operating expenses could quickly get you into trouble if the FAA disagrees with your math on that.
Now if the "friends" are actually members of the general public, then yeah that would really need to be 135. But if they're actual friends, then I think the standard would be don't achieve net positive revenue... let them pay for gas and contract pilot only, that's easy to account for. If you start trying to account for fixed overhead costs (lease, salary pilots, some mx), that seems problematic.
Do a search on the FAA website for legal opinions, or write them a letter and ask for an interpretation...
https://www.faa.gov/about/office_org...terpretations/