Parakeets are almost always friendly with each other if all placed in the cage at the same time. Just use common sense and allow the birds some elbow room. Don't over-crowd your birds. If you have had a parakeet by itself for some time, you might be thinking of getting a second bird as a mate or friend for the first. This is a good idea, but don't put the birds together right away.
Birds are just the same.
Put the new bird in a different cage next to the original pet. When you see them playing through the bars, they can be placed together. Just to be on the safe side, do keep an eye on them anyhow for a day or two.