Four-Play: Seeking Shallow End Advice

My wife and I are considering exploring our (first ever) foursome with a couple we’ve known for a long time. We’ve traveled together plenty, and occasionally the sexual tension has been palpable - skinny dipping, touching, etc. We’re vacationing with them again this summer, so the other day I asked the husband if a foursome is something they had ever considered.

He expressed interest and I shared this with my wife. Three of us are ready to jump in. His wife is more reserved. We all think it’s something that might interest her, but we want to be respectful / not manipulative, yet safely let her know our thoughts and see if she’s interested in wading into the shallow end with us.

Wondering if something like a sexy game of truth or dare might be a fun litmus test. Perhaps some other activity that allows us all to explore this at a slower pace for her comfort.

Has anyone successfully navigated this scenario before?

Any advice / suggestions / recommendations?