parakeet helph

i have 2 at the moment they are a couple and get along pretty well and are friendly. i had them over a year i want to add more to them possibly 2 or more how should i do this do i just add them to the cage or wait a couple of days or this is a bad idea.. anyone who has added another bird to the ones they already had how did u do it.. and what should i get i wanted to get 2more females as i have a female and male at the moment or should i get another couple a male and female please help me out i have a huge cage.