We have gone through this before with my grown daughter. She has visited for a few days while we are at WDW. The first 2 times we just went to the front desk and told them she would be visiting for a few days, and to charge us as needed. They never did charge us, and one CM even told me to just add her as a day guest and she wouldn't be charged. Some pixie dust for us once.
For our trip this March, my daughter is going to visit again, but this time for 5 days of our 8 day trip. We love having her with us and she gets a break from the cold northern winters. However, this time she decided to set it up with our TA. She is on our flight down and is registered as a guest in our room too.
She is on our registration so she can have adr's and fp's set up with us. She just has her flight back home on a different date than us. We always use ME so that's a big help for her. Before, she had to set up a town car service to and from. She also had different fp times, if we couldn't coordinate. This way is better, but more expensive for her. She feels more comfortable being a registered guest than just a visitor for a few days (on the registration).
Hope this helped and wasn't too confusing. I'd be glad to answer any specific questions.