There are a few species that are considered to be communal or somewhat communal.
There are also keepers of these species who keep them individually and recommend doing so.
Depending on who you talk to, there may be no species that are communal.
Mellerii can be kept in groups with a proper (large) set up that allows individuals to separate themselves from each other.
A couple of others would be:
Rudis
Some pygmies
Keeping any species of chameleon in a group situation requires a good deal of experience and an ability to "read" the animals.
It is almost never okay to keep males of any species together.
All species can be kept well by themselves.
-Brad