Adding people to a reservation cannot be done online. I added folks to a reservation that I had made online. I could change arrival/departure dates online but not adding people. You have to call to do that.
As others said, since you are inside 30 days, just get gray Magic Bands when you check in. Or you can buy MB's at the
Disney Store with various characters/designs and link it to that individual in
MDE.
If your new guests have their own MDE account, there is a way for them to be linked to yours for purposes of meal reservations, FP+, etc. Or you can add them as additional people on your MDE.
This is one reason I hate MyMagic+. It adds additional crap when plans/people/dates change.
Bama Ed