Honestly you need to let these guys know that while you totally support the idea of tripping together you need to take some necessary precautions.
First of all to check that deficiency. If these guys have ever at any point taken over the normal amount of cough medicine, they'd know if they were off. People report crazy trips with 100mg when they are deficient. My friend sips his cough syrup off the bottle when sick, so I know he was fine.
Then you should really get a 2nd plateau experience under everyones belt. Do it together. Its tons of fun. I had a group bash with everybody measured out to 6mg/kg. We all had a great time. Just dancing to music and watching tv and staring at things.
And trip sitting can be boring as hell if you arent on SOMETHING. So do some yourself. I find I can walk around, pee standing up, and get water at around 6mg/kg. Beyond that, those tasks become quite hard.
Ultimately, the truth is 3rd plateau does not lend itself well to group settings. It may vary in your case. But I basically tried to treat my first 3rd plateau run the same as a normal 2nd. Music pumping, friends over, goofing around, and I got beat by the dex gods.
The second time I took a much more relaxed calm approach and just layed there and floated through dimensions unknown to my person.