You ask your dog to move so someone can sit down. They go still, then growl. The growl is uncomfortable to hear, but it is a warning you can act on before anyone is hurt. Do not grab their collar, push them, or punish them for making a sound.
Give them space. Keep children and other pets away from the sofa until everyone is settled. If your dog can be called away without coming near them, invite them to a different room and reward from a safe distance. If they stay put, let them be for now and manage access to the room later. Do not corner a dog to prove the sofa belongs to you.
Check what changed
A sudden new growl can signal pain, trouble jumping down, or another health issue. Arrange a veterinary check, especially if your dog is older, stiff, or sensitive to touch. A dog may also guard a comfortable place because they worry it will be taken away. Either way, the training plan starts with safety, not a contest over furniture.
Give your dog a comfortable bed where people won't disturb them. When they are not guarding the sofa, teach an "off" cue with an easy, willing step down and a reward on the floor. Practice only when your dog is relaxed, and do not lure them close to someone they are worried about. Reward them for choosing their own bed too.
Until you have an individual plan, prevent the scenario when possible: close the room or use a gate when your dog is elsewhere, and set clear household rules so nobody tries to move them by hand. If there has been a snap or bite, if children are involved, or if guarding spreads to other places, work with a veterinarian and a qualified, reward-based behavior professional. The sofa is replaceable. A warning ignored can have a much heavier cost. For more on guarding a resting place, see the resource-guarding guide, do not reach for the dog or try to make them prove the sofa is yours
Sources: Dogs Trust: How to keep your dog off the furniture. Dogs Trust: Resource guarding in dogs.