I hear the two of them have been together since last year.
Tips
grammar
一起 goes before a verb: 一起吃饭 (to eat together). 在一起 comes after a verb or stands on its own as the predicate: 住在一起 (to live together), 我们在一起 (we are together).
usage
在一起了 on its own means a couple has started dating, which is how people announce it: 我们在一起了! 'We're together now!'