For example a whole crowd of men surrounded and beat someone up.
closed as primarily opinion-based by FumbleFingers, TrevorD, Nathaniel, Ste, ab2 ReinstateMonicaNow Jun 8 '16 at 19:53
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I'd say mob is your best bet.
A mob surrounded him and beat him up.
You could also use the word; horde.
A horde is a disparaging word for ‘a large group of people’, as in hordes of fans descended on the stage. Instances of hoard being used instead of horde are not uncommon: around a quarter of citations for hoard in the Oxford English Corpus are for the incorrect use.
Oxford Dictionary of English