Used as a noun or の-adjective for things that are the same, such as dates, places, opinions, or items of the same kind. 同じ is the more general equivalent.
一緒にするdo together; do something jointly · put together; lump together; mix; join · regard in the same light; put in the same category; confuse · arrange for (two people) to marry; bring together as a couple
一緒になるcome together; join; merge; meet up · get married; become a married couple
味噌も糞も一緒lumping good and bad together; treating everything as the same
糞も味噌も一緒lumping good and bad together; not distinguishing between good and bad; mixing the worthy with the worthless
盆と正月が一緒に来たようas if two great things happened at once; as if Christmas and one's birthday came together
The baby started crying at the same time the phone rang.