Presumably you're referring to people, so the answer is what mikehj wrote. lehakir is used for knowing people, ladaat is for knowing things, knowledge, subjects etc. After both verbs you need to use "et", in its appropriate form, oti, otach, otam, etc.

Just a small pronounciation correction: אתה מכיר אותם? ata mekir otam? (not makir!) It is not "hayadata hem". Technically, it would be correct to say 'hayadata otam". You will be understood but that sounds like obscure Hebrew. Even somewhat biblical. The verb ladaat (to know) is translated better for knowledge rather than to know people. For example "hayadata she-Rachel po?" Did you know that Rachel is here?

is not " hayadata hem"?

אתה מכיר אותם? ata makir otam?