The game is balanced for those that like group play and those that like solo play, at least until you get farther into the game and doing dungeons/raids/trials pretty much becomes all you really do. Before that, how much you play with others and friends is really up to you.
It is really only very specific quests, like the Class/Job quests that are every five levels that require you to do them by yourself. Asides from that you can group up with friends for everything else.