Some games require grouping. We hate that, especially when certain classes are required, because you can easily spend half your in-game time looking for group members. Some games encourage soloing. We often like that, but single-player games deliver a much better solo experience. Some games discourage grouping, often as an accident of game mechanics, which is just poor. Some games encourage and reward grouping without requiring it, which is the best of all possible worlds.
I have a very long version with many examples after the break, but that is the core of my message today: encourage grouping, do not require it, and make sure the game mechanics really do encourage it.
You encourage grouping by increasing rewards for groups and adding abilities that require groups to take full advantage of them. You require grouping by giving enemies ridiculous numbers of hit points, failing to scale encounters for different numbers, or making encounters that demand (or all but demand) several specific abilities that are spread across the classes. You discourage grouping by making quests difficult to do together and failing to scale encounters for different numbers. Yes, a lack of scaling can both require and discourage grouping.
