All defendants on the plaintiff's claim or all plaintiffs on the defendant's claim must be served. It is not enough to serve one defendant or plaintiff and assume that that person will tell the others. This is true even if the parties are married, live together, or do business together.
If you don't serve a party, the judge will postpone the hearing or dismiss your case against that party. If you sue only one person and don't serve that person according to the rules, then the judge will probably postpone the hearing until after you're able to serve that person. If you sue more than one person and serve only one properly, the judge cannot enter a judgment against those people you didn't serve and will dismiss your action against those parties. Some states will allow you to refile your claim against those parties if you wish.