I've done a large (~15 guys) anonymous jerk off club before, it was fun.
However, my favorite experience ever jerking off with someone else was with one of my good friends. He's straight, athletic toned body. Early in college, we started jerking off together while watching porn and did so pretty regularly for several months. I liked how he was always open to comparing our penises and joking around about it. Plenty of other good memories and stories from those days, but my favorite time with him came years later.
We went the last couple years of college and phased out jerking off together. Then moved away but kept in occasional contact. When I was 28, he came to visit me for the weekend. First couple days were filled with catching up, and going out together. Then one morning, we were finishing our coffee, about to start our days. He said he wanted to go jerk off (implied alone) before he showered. I told him that I had a box of some toys he should try and he was very intrigued. We went up to my room, and I showed him my secret stash of some various vibrators, cock rings, and penis pump. He was kinda taken aback at first and did know if he wanted to try. I kept pushing him and said, "here, I'll show you."
With that, I slipped down my pajama pants. My penis was soft and a little shriveled at the time. I grabbed the penis pump and said, "Watch this." Lubed my penis, inserted it, and started pumping. Soon, my penis was engorged inside the plastic casing. First time he'd seen me erect in probably 9 years. "Wow dude, look how big that vein is," he said, pointing to my penis through the plastic. I deflated, and finally talked him into trying. He sat down on the side of the bed, inserted his penis, and I helped him pump it up. Soon enough, his penis too became very engorged.
We moved on to some dick head vibrators and finally a vibrating cock ring. He left that on his penis while we searched for some porn. Next thing you know, he pulls up a video on his phone and says, "watch this." It's a POV of him getting a bj from a girl. Talk about something I've dreamed of seeing before! He'd always joked that he think I have a big penis, and started again commenting on how big I was. I grabbed a soft tape measure, and we measured up to compare. I was 6.5"x5.5", he was 5" x 5.5". So hot getting to compare penises with a close friend like that.
We ended up getting off (we've never touched each other, just mutual jo), then went ahead and took our showers and proceeded on our day. I think he needs to come back for another visit soon.