Because the posts go in chronological order. If I've just posted Carr in a bar on Tuesday, I'm not going to tag him in somewhere on Friday, because that means I can't do anything on the Wednesday or Thursday. Your best bet is to look at the last few posts on the community, and date accordingly.
Ok, well if they're new to the city, they need to meet people somehow. There's pups that own stores - I've got Lex with his comic book shop, and there's also the garage, and bookstore. You could choose those, or a cafe near any of the businesses.