Depends what you mean by "visit." Can they come play cards in the room? Sure. Can they use the pool? Not officially, unless you have room and add them to your reservation.
Not hang by the pool......was hoping to ask her if she could bring a cake for DD 16th bday (as I cannot see spending the $59 for a 6" Disney cake) and have her celebrate with us in the room. Just wasn't sure how she would get past the gate (I suppose I let them know??)
Surely they can still get through to see family, I hope that hasn't changed. I believe your cousin will need to provide a photo ID and tell the guard who they are visiting. Telling him about the in room birthday party will help.
You should be fine-we had relatives visit us twice at SSR in May. You are allowed visitors they just are not supposed to use the pool if they aren't on the room reservation.
We actually had added them to our BLT reservation (had a split stay) since we had room and were afraid they might not be allowed to park at CR/BLT-did not have any issue at SSR.