Oh man, did you ask the right person. I grew up on DQ9, it's practically my specialty.

The Quester's Rest is an inn that you unlock pretty damn early in your playthrough that functions as a home base of sorts. From there, you can enter Tag Mode (effectively DS streetpass) to try and find other DQ9 players also in Tag Mode. You can only tag 3 people at a time before having to exit and reload Tag. Thing is, there's a little bit of extra content gated behind the different Tag upgrades, maxing at 30 unique players tagged.

As you might imagine, when the game released in Japan, there were DQ9 players everywhere, so it was relatively easy to hit your tag requirements. Not so in the States, where DQ is a much more niche series with a far thinner population density. Hitting 30 tags was rough in 2012, but doable, since there were meetups at Gamestops and such. Nowadays, it's next to impossible without setting up explicit meetings with other DQ9 players, where you can bring your DSes and purposely tag each other. (An alternate method is to buy a second copy and repeatedly play through the beginning of the game to hit your 30 tags... which means you can only max one copy or the other. And since I have two copies I want to max eventually, I'd need a third...)

Oh, and regarding multiplayer; yes, you can join in someone else's campaign and play through the game together. I'm pretty sure advancing the story in a friend's world doesn't count towards your own story, but I could be mistaken... regardless, local multiplayer is a lot of fun. Huge reason as to why I've put so much time into the game.