It never hurts to ask if anyone is interested in doing a dungeon. Not only are L50 players often bored of doing the same thing over and over, but many of us are likewise keen on partying up with other LS members and it can only become easier by knowing when they are going to go and do a dungeon. I always mention it before I hop into one, and I've not done a single L50 dungeon yet (soon... I'm eight quests away from being done with the main storyline).

Having said that, I see people all the time. I've passed almost everyone in the LS at some point or another - or at least it feels that way! I was gathering with Valerna the other day, I've partied with about ten people so far in duties, I always see Tony in the Ul'dah markets and often see Yosefu there, too. I probably pass Nolwe once every 1-2 days in all kinds of places. I pass Yeargdribble every now and then, Ceridwen too. I will say that you don't get to party up with people who have the same role as you very often if you're a healer or tank, but I still manage to stop by them once in a while as I roam the world. But then, it all depends on how much of an eye you have for spotting the <FC> bit next to people's names.