From my experience with multis, newcomers are NOT welcome at all. My 4 killed a female I tried to add by not letting her into any shells, and when she finally hid inside a shell, they wouldn't let her out. She went so deep inside the shell, she got stuck and died
I had the same experience with my multis but they will protect the young and the colony will grow. 1 thing tho when you go to the lfs there will most likely be no shells in the tank because once they have shells the only way i know to take the shelly's out is to use a container lower it into the tank and move the shell into it. This way there will not be an air pocket in the shell. When ever you put a net or anything in the tank they will shoot into a shell.