Cats, contrary to what we often think, are by nature not particularly social creatures and usually have no need for a new companion. So you cannot force your cat to welcome the newcomer enthusiastically.
Occasionally, the introduction of a new cat goes smoothly and they are immediately best friends. Other cats will never like each other and there is always a tense truce, which can occasionally get out of hand.
Unprepared introduction of a new conspecific often causes stress, which can lead to aggression, unzyness and other unwanted behaviour.
Iscavets Veterinary Practice has prepared a brochure with a bunch of useful tips to increase harmonious cohabitation over time. Living together harmoniously does not necessarily mean that they become best friends, but it does mean that they can spend time together peacefully and relaxed.