"Among themselves" or "among them"?
I would suggest that neither ending is needed. Isn't it sufficient to say:
...where successful people are giving lectures and people could exchange knowledge.
I don't think among them/selves adds anything useful. Who else would they be exchanging knowledge with? People that weren't there?!
But if you must have an ending, what about:
...where successful people are giving lectures and people could exchange knowledge with each other.
As an aside, I think I would have said amongst themselves if using those words.