I like my sims to have friends. Some more than others, but I try to let everyone have at least one friend.
It's much easier to get them interacting if they are gathering in a club meeting. And if you want them to eat together you can indeed cook and call to meal. This includes at outdoor bbqs at the park etc. Or, if you don't mind paying, treat them to a meal at a restaurant.
At bars you can buy drinks for other sims. You can ask them to join you in activities like roller skating and bowling.
If you set up a club for them, you can choose club activities as well as things that you want them to avoid doing.
And when your sim is down, they will often get a friend come round to cheer them up or invite them out.