彼らは自然と急に笑い出した。

they (usually male); them

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

nature; spontaneity; naturally; spontaneously

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

urgent; sudden; abrupt; sharp; steep; precipitous; rapid; swift; fast; emergency; crisis; hurrying; haste; (in gagaku or noh) end of a song

at (place, time); in; on; during; to (direction, state); toward; into; for (purpose); because of (reason); for; with; by; from; as (i.e. in the role of); per; in; for; a (e.g. "once a month"); and; in addition to; if; although

to laugh; to smile; to sneer; to ridicule; to be dumbfounded; to be flabbergasted