It's a big deal for an attorney to be formally representing a person or legal entity. It's often better not to have that formal relationship in a complicated situation where lots of people are filing complaints in lots of different courts. Note that Lin Wood is also not part of Trump formal legal team, and he's probably not on the informal team either. Sidney appeared at the press conference with Giuliani and Ellis. so she's at least been working with them informally, but that does not mean she's formally representing Trump or his campaign organization.