I'm guessing most people who use these forums and also play the Sims Mobile will already have figured this out, but I still attend a lot of parties that stall out around level 5 or 6, so clearly there are people out there who either don't know or don't care.
The short version is that you can use the green social interactions (as opposed to the party-specific purple ones) over and over again at no energy cost, gaining you 25 party points each time. Yes, it's a grind, but it's also 100% free and you have 24 real-time hours to do it in.
Why bother? It's one of the very few possible ways to get the best sim tickets--the purple 'luxe' ones. Two per party, unless something is bugged and you don't get them (which unfortunately happens far more than it should, still).
So far I'm enjoying this game, in part because even though I have yet to spend a cent on it, I feel like I am able to progress well. That's exactly what I look for in a mobile game, and that's the kind of mobile game I'm actually willing to spend a little money on, at some point. I just can't afford to play games that are constant money sinks.
