I strongly prefer tripping alone to tripping in a group. This is especially true nowadays since most of my psychedelic-using friends are a lot less experienced than I am, so most of the time when I group trip I end up having to tripsit while tripping, which isn't much fun. I don't mind tripsitting for friends, but I'd rather just smoke some bowls, maybe partaking in a little DMT but skipping the long-lasting psychs.
Alone, you never have to worry about other peoples' trips - if you feel like changing the music, you just change the music, if you feel like changing settings you just stand up and start walking, etc. It's great for quiet introspection and all that, too, but even for a less 'heavy' and more recreationally oriented trip, I prefer to go solo. Conversations can be fun sometimes while tripping, but I'd rather have them online or over the phone, so I can start - and more importantly, stop - socializing on a whim. My favorite group tripping experiences are all from back in college when I lived in the on-campus dorms; I would start my trips solo and experience the peak, then meet up with friends (esp. if they were also tripping that day) to smoke some bowls and transition to a group setting. The most memorable instance was a high dose LSD trip; I went outside to smoke a cig, and ran into a good friend of mine who also took a bunch of L that day. We hadn't talked in a few days so neither of us knew the other was tripping that day until we ran into each other, both all saucer eyed and spouting nonsense... We took a walk while smoking and chatting, then I started peaking harder than before and sorta lost track of the conversation while my ego dissolved... Later that night we met up with a bunch of fellow hippies down by the river and smoked some salvia. Good times. </barely relevant tangent>