Hi Lucy,
Welcome to YT!
I have 4 yorkies, my oldest is 13 yrs old name Buffy then there is Duncan 4, Keanu 3 and Bear 3.
I never gave it a thought about how to introduce a new member, I have just brought in the new pup and put everyone in the yard to smell each other with some toys and treats.
It does not take long for all to start running more like a show off of who can run the fastest...lol Oh except my girl she's too old to run like that, unless she hears the car keys and shes in another room then she will push the boys out of the way...
Good Luck!