How to have a great group outing?

You can have a great group outing with only two sims and without leaving your home lot.  Having only one guest is easier, as you only have to account for the needs of that one sim.  I'm not sure about the time constraints, but the minimum is probably two to four hours.  If the outing is successful, you should get a small positive (I think +5) moodlet from it.

The only thing I worry about is maxing the other sim's fun meter.  TV takes care of this, as does a movie.  If the sim is hungry (look for the thought bubble to display any negative moodlets), they can eat from the fridge if you're home, or you can take them to dinner.  If the sim is "having a blast" and has no negative moodlets, I always get credit for a great group outing.

One other thing that helps but is not necessary is having your guest make a friend during the outing.  If a sim has achieved the "Celebrity" charisma challenge and has bought the Attractive LTR, a friendly introduction will produce an instant friend.  So if you have another sim in the household who meets these criteria, you can give your guest, and your sim, the new friend moodlet as well, with barely any effort on your part.

I don't see why not, although I haven't tried it in a while.  I imagine it would be even easier to make sure everyone's happy.  But if it's not working, maybe there's a requirement for the group to actually go somewhere; a guest would have already traveled to your home lot.