This phenomenon has a very simple definition while it can be widely experienced by everyone in the world nowadays. Deinviduation is basically acting in a group of people differently that would act by yourself. This trait is somewhat different to what is known as Self-monitoring. The latter is referred as behaving according to what the group is pushing an individual towards, it has more to do with acceptance. In the case of deinviduation it is also defined as loosening normal constraints on behavior, thus leading to a tendency of impulsive and deviant behavior. The interaction as a system of people inside a group moves people to act differently.
As far as my experience when attending high school I can report that I really felt much more free and excited to go out and have fun in parties. I definitely couldn’t hide my excitement, and that was expressed through dum jokes and goofy behavior just because I was surrounded by my best friends which had a lot in common with me. Therefore I felt no constraints to my behavior, in fact I felt more liberty to just have fun. This happened all the way from my freshman to senior year before I graduated from High School.
In sum, I can attest to the fact that this really happens. For me at least, I didn’t have to be or feel anonymous to behave in a different manner. To put it simply, I felt happier and in peace with myself when in groups. If there are any cons about being in groups I would categorize them as minimal compared to what you can learn about yourself when interacting with others.