Off means to remove your paws from whatever surface they are touching. That could be furniture, objects, or people. This command is not to be confused with Down. Down means to lie down. If your dog is lying on the couch, she is already performing a Down. If you want her to get off the couch, the command is "Off".
Relationship based training philosophy
Off is really simple. It's harder to teach your spouse not to say Down when they want an Off than it is to teach your dog "Off"!
When your dog has her feet on something, say "Off", and genly guide her off the object. If your dog shows aggression when you try to physically guide her from furniture, use a treat to lure her "Off". Do not try to use force with her as it could lead to a confrontation that neither of you wants. As soon as she has four on the floor, praise and reward her. After several repetitions of this, say "Off" and then wait a few seconds to give her a chance to do it herself. If she does, praise and treat. If not, go ahead and guide her. Eventually, you will be able to say "Off" from across the room, and she will comply!
Get Off ME!
If you want to make sure that you never have company again, let your dog jump all over your guests. But, if you actually enjoy having visitors, teach her not to jump. Off means paws off whether it's objects or people. The challenge with teaching Off with people is that some people like having a dog jump on them. They say, "Oh, it's ok. I don't mind.", and then they pet the dog. Dogs do what works. So, if a behavior works for them, they will repeat it. Just one person petting your dog just one time for jumping up will undermine your attempts to stop her from jumping up on those people who DO mind. It will be harder to teach your friends not to reward your dog for jumping on them than it will be to teach your dog not to jump on people! Ask your friends instead to say "Off", then take a step back so that her paws go back to the floor. As soon as all four paws are on the ground, praise and treat. THEN, your friend can pet your dog. The rule is, ONLY four paws on the floor gets attention.
Note: jumping up on people is a dog's way of trying to solicit attention. It is not an attempt to "dominate" you. Please do not punish your dog for trying to be friendly. Reward her for appropriate displays of affection instead.