System names aka body names
There's a great deal of difficulty talking about a single-bodied entity as more-than-one PERSON. People in the multiple community trip over language, as do the people who know and love multiples, all the time. Words are coined to help out with the awkwardness of English with a first-person-plural, and people who are many-in-one-body will often come up with nicknames (aka "use names") to help get around the awkwardness of a single birthname or legal name.
Names can be powerful, awkward and useful concepts. Almost everyone knows at least one person who absolutely abhors their birthname, often using a nickname, or their middle name, instead of the one their parents gave them. We all know of Madonna who born "Madonna Louise Ciccone" and no longer uses her middle or last name, Sting was born "Gordon Sumner", and Engelbert Humperdink was born Arnold George Dorsey, and recieved the nickname Gerry Dorsey by fans before settling in on his present professional name. There there is Prince, who also became "The Artist Formerly Known As Prince" because no one could pronounce *Symbol*.
Some multiples do not want to hear their birthname. Sometimes the birthname is associated with a single entity within their head and other entities do not associate with the name. Sometimes no one in the body associates with the birthname at all.
To avoid confusion when talking about the group of entities when only the birthname is available, one convention is to add a terminal "&" to the name, such as Joe&. This is pronounced "-and" as in "Joe-and", and was created in this community as shorthand for "and company" as in "Joe and company", and was first written as Joe & Co. This convention is also used when the birthname does not apply to the entities-inside if the birthname is used to signify the group, because it automatically implies that Joe is not a singleton, thus it's a group of entities in one body.
Another convention is to take on a body name that is different from the birthname. Often these names naturally signify a group entity, such as (forgive me if anyone is using these) The Battalion, The Herd, The Circus, Gaggle, Motel 6. There are The Crisses, for example, a name that is already pluralized from Criss (i.e. "Joes"). There are simply words used as names, such as the body-group who was formerly known as "Dandelion". All of these names connote the body and at least a subset of the group living inside of it... "Any or all of the entities residing in the body going by the name of..." Context will usually denote whether it means everyone in the body or not. For clarity, one may ask. Using a pluralized or group name negates the need for a terminal "&".