There is a popular myth regarding the “pack” mentality with dogs, and many professionals have debunked it.
For example, you have a super excitable young dog or puppy who LOVES to play with an older dog. Your older dog is very tolerant and allows the dog to push many limits, but you're concerned that the young dog will go too far. You can reinforce polite play by continuing to allow the play (reinforcing the good behavior) or pull the dog out for 1 min time out until they settle, which punishes the over-arousal or bullying play behaviors by taking the 'play' away.
I have many dogs in my household at any given time, so I know the difficulties, but I also know how great it can be to have a group that all gets along!"