How would you say "they know them"?

Conocer is best used when you are talking about knowing somebody or some place.

You'd probably want to say
"(Ellos/Ellas) los/las conocen".  Term in parentheses is optional. Use "Ellos/los" if they/them are male or objects where the gender of the noun is unknown. Use "Ellas/las" if the gender of the noun is feminine.

If "them" is not a person or place, but an idea, then
"(Ellos/Ellas) los/las saben", using the same rules as above

Here's a little information about direct objects in Spanish.

*insert 'I'm a novice disclaimer' here. Please correct if I'm in error :)*

Hello neophyte:

Please give us some context. What are we talking about?. Depending on your answer it could be "lo saben" or "los conocen".

Saludos!

Lucho2007

 

 

Ken In San Jo:

 

Los (las) conocen. Conocer is used for peolple and places. Saber is used for other things. Por favor, los nativos, ayudamen.

You may want to visit this website. I think they do a good job in defining the concept.

"saber
to know a fact, to know something thoroughly, to know how to do something

conocer
to be acquainted with a person, place, or thing"

[link]