With

"With" can be translated as 함께, 같이, or 더불어.

If you're using a verb that describes an action that is individual, such as 가다 (to go), then you use 같이, which says "with" as in "happen to do at the same time".

If you're using a verb that describes an action that requires many people working together, then you need 함께. This implies working together, at the same time, towards the same goal, in unison.

If you have a verb, like 살다 (to live) that could be independent or together, then the word you use will change the meaning in an important way. "같이 살다" means to live independent lives in the same place and time; "함께 살다" means to live supporting each other.

더불어 seems to be a more neutral term.