means everyone or everything together.
If the two terms can be separated, that’s a dead giveaway that the term you want is all together.
is an adverb and means "all in all," "all told," or "completely."If you can replace the term with something like "completely" or "when all is said and done," you are altogether better off with altogether.
If you can rewrite the sentence to use all and together separately, the term you want is all together.