"them saying to me that..."

I watched the interview of President Obama and he said "I remember having a conversation with somebody, and them saying to me that, you know it's not about you. It's about what you can do for other people."

I can't understand the grammatical structure of "them saying to me that..."
Why didn't he use (they were saying to me that...? Could you tell me when you use "them" as a subject of a sentence?

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@monimonya
"I remember having a conversation with somebody, and them saying to me that, you know it's not about you. It's about what you can do for other people."

I think I have the answer. Them in this sentence refers back to the object - somebody. I think this is why it sounds right to my ears.

They is used to refer to the subject of a clause. In other words, it usually represents the ‘doers’ of the action described by the verb, and usually refers back to two or more people or things that were mentioned earlier.

Them is used to refer to the object of a clause. In other words, it usually represents the group of people or things that have ‘experienced’ the action described by the verb, and refers back to two or more people or things that were mentioned earlier.

When you ask about their- you mean they’re or they are saying to me.

I hope this helps, if not I can keep looking for a better explanation for my crazy language.