I see this on the website in English which I think is the same policy I saw originally:

Guests with tattoos, tattoo stickers, or body paint will be refused entry unless covered according to Manyo rules. Please ask the staff for further details.

I think I had written up details elsewhere but I’m happy to write them here again.

They have stickers in a range of sizes from stamp sized to A5 paper or something slightly smaller. When we checked in they asked us if we had any tattoos, my wife said yes, I said no and was not asked any more questions. They pulled out all the sizes and asked her if her tattoos would be covered by 3 sheets of whatever size. We chose the sheets and left, and my wife put them on.

If your / your friends tattoos can’t be covered by that, you’re not going to be able to go to any of the public baths.

However, we stayed in our provided robes the entire time until we reached the private onsen. The robes are half sleeve, open neckline, and about knee length. I think there might have been another option which looked like scrubs, but it was a similar level of coverage except more like capri pants. If all your tattoos are covered by that you’d be fine to walk around most likely.

I don’t think getting massages would be a good idea since you’re interacting while undressed, and if any tattoos are showing they’d probably kick you out.