What is opposite of disperse?

I cannot think of a one Word antonym (Word with the opposite meaning) to "disperse". However, to disperse some "thing" means to distribute that thing widely. Thus, a group of people can "disperse" after meeting together. Obviously, a "single" thing cannot be dispersed - one cannot speak of an elephant being dispersed. So one can only disperse collectives (groups or sets).

If asked for an antonym to "disperse", I would say "aggregate", or "bring together" or "concentrate" (as in "many people are concentrated in this part of the city").