When watching Multiple Dogs. How Do you Introduce a new dog to the pack?

I have two dogs (one of whom can be dog-reactive on leash - hyperfocused, bossy, and defensive of her space) and I watch up to two others at a time. I always do introductions one on one outside, first with my mellow girl, followed by my more challenging girl, and don't allow the dogs to sniff each other more than a couple seconds before separating them, trying to time it so neither dog gets nervous, tense, or snarky. Then we take them for a group walk, at first preventing the dogs from interacting with each other, and slowly allowing them to sniff and walk side by side as they settle down. By the time we've gotten back to my house, the dogs are comfortable and calm with each other 95% of the time. I allow the client and their dog to come inside at this point and let their dog explore while either keeping my dog on leash, in the kennel, or under verbal control, not allowing my dog to pester the guest, but allowing them to interact with each other should they both want to.

When they are dropped off for the stay, I usually have my dog(s) kenneled so the new guest has the opportunity to investigate everything on their own terms, then take them all out for another group walk. After that, everyone is very closely supervised and separated as necessary if someone starts getting a little stressed or overwhelmed with the group. In most instances the dogs are completely comfortable with each other within the first day. I've had only very minimal conflict with the dogs I've watched and no injuries, and I've only turned down one client due to dog incompatibility, which might have been manageable, but I did not deem it to be worth the effort or the stress on my dogs. It's their house, too, so I'm not going to put them in an uncomfortable position if it isn't necessary, and since their social skills are generally quite good (minus a couple quirks that we actively work on), I don't see the need in forcing them to spend time with a dog they just don't like in their house. It's worth pointing out that the one dog I turned down did not want to do a group leashed walk during the M&G due to leash reactivity with her own dog, so in my experience this method of using gradual introductions and group walks has resulted in a very high success rate.

This can be stressful, at least for me at times. I cannot introduce new dogs on neutral ground. What I do is the dog that is most excited I put on a leash while all the dogs go out to my big backyard together. I think because they are in an open space it's a lot less tense for the dogs. Always, the dogs need to go potty anyway, and usually after that and some sniffs all is well with the world.

When we adopted a new dog and had to introduce him to our other 2 dogs we did somewhere none of them would feel threatened. We introduced them one at a time and kept them both on leashes. You let them sniff each other for 2-3 seconds at a time, pull them back, praise them, and repeat w/ all of them!

Personally I do slow intros just to be sure that if there are going to be clashing personalities appropriate steps can be taken to ensure the safety of all. If the current dogs have crates I put them in their crates to create a barrier and to give a bit of time to settle and smell without out having somebody immeadiately in their face. If a crate is not available or I have just 1 dog then I leash them and still make sure it's as slow and controlled as possible. I have never had an issue doing it this way and seems to be a big help anxiety wise.