Please take a look at the picture of this news article :
Those people are gathering together.
Imagine you're one of them.
In such a case you would say the above sentence :
"Saya berkumpul dengan mereka" ('I get together with them').
Take a look at the picture of this news article :
The President is shaking hands with some people.
In such a situation, the President could say:
"Saya bertemu dengan mereka." ('I meet with them').
That's the the difference in ID between "bertemu" (to meet) and "berkumpul" (to gather).