Some friends of theirs means some of their friends. As you can see, theirs is a possessive pronoun used for them. In English, instead of saying one of my/her/his/our/your/their friends, we say a friend of mine/hers/his/ours/yours/theirs. So in the senternce "Here are my parents with some friends of theirs", we have to use theirs instead of them to show possession, so to speak. Using 'them" would make the sentence very confusing.
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