If I am not wrong, I once encountered a British word used (in slang i suppose) to describe a person who boasts about going out, knowing or dating famous people. Does it exist? In this case, is it common? Thank you.
Are you thinking of name-dropping (MW)?
the studied but seemingly casual mention of prominent persons as associates done to impress others
Name-dropping is the verb form, the noun is name-dropper.
If your use of "knowing" is the biblical sense of the word, there is also the term "starfucker".
The Term "Groupie" is often used in this case.
While, most commonly used in reference to a band/musical artist, it can also be used in context with any famous person or group.