20mg sounds like a safe dose for everyone to me, especially since you'll be in a comfortable environment. Unless one of your friends is pretty sensitive to psychedelics in general; I suppose they have experience with other substances? Worst thing that will happen is that you'll get disappointed because of a high (natural) tolerance. My first and only time on 2C-B so far I took 24mg and only got slight visual effects (they were nice though) and the MDMA-like feeling (actually it was better than my few MDMA experiences), as DickTate described. I just have a high tolerance to 2C-x's, I already knew that from my experiences with 2C-E. Didn't get any noticeable nausea or bodyload though, took it orally in pill form (1.5 of them, to be precise).
Never plugged 2C-B, but I've done it with 2C-E and that was a success. The worst thing about it was the fact that I shot 2ml of liquid and some air up my butt (first time plugging), which was kinda uncomfortable, but other than that I really can't complain. Compared to snorting the bodyload was like cuddling a huge stuffed animal. I suppose it's somewhat the same with 2C-B, although I'm not sure if it gets twice as strong. Is it stronger when snorted? If so, it'll probably be stronger when plugged as well, so keep that in mind.