it will be mostly full of people in small groups who already know each other and went there to talk to each other

doesn't imply “no-one ever meets anyone new”, though it's not obvious from outside observation.

nobody goes to such a place alone

doesn't imply that they're not interested in meeting new people; indeed, it is easier to meet new people when you're not alone (social proof i.e. he's not enough of a weirdo as to not have any friends, having a wingman reduces anxiety, etc.). Also, logistics (if there's several of you, you can share a taxi which is cheaper than taking one alone, or if someone has a car they can be the designated driver, etc.)