Trivia works. I go to two trivias (M/W) almost every week. I have a group of about 6 people that show up (random which subset) to each (not the same people) to play on my team, sometimes more (used to be a lot more but some people moved). And the other teams are people I knew before going there or have come to know since starting. Pleasant way to spend a couple hours a couple times a week. A bit of hanging out, a bit of competitive fun, and more hanging out.
I'm pretty welcoming and gregarious, I have had random people join my team before when they asked (my teammates were less comfortable at first, but relaxed and welcomed them later). This will definitely depend on the crowd. It may be easier to start your own team (or playing solo at the bar) and just start getting to know the other teams for a while first before asking to join them.
It also depends on the location. One is a restaurant, people are generally less welcoming to random folks sitting at their dining table (unfortunately). The other location is a bar. It's much easier to just join a random team. Find 3 people sitting at the bar and ask if you can join them. Or, like the last folks that joined us, find a table with several empty seats and ask to join them.
