You will be informed in your MDE account that you will have until about a month before your arrival date (the site will actually give you a date certain) to personalize the bands (choose colors and add names to them). The bands will then be automatically sent out right after that date given in your MDE account.
Yes, it is a good idea. And if you add each of your family member's tickets to MDE then you will be able to do FP+ 60 days before you check in. Also MDE works real well to make ADRs as well.
If you want to customize (select color and name for underside) your MagicBands and make advance FP+ selections (must have tickets linked to MDE), yes you need to set it up and link your reservations and tickets/passes (if you have them already) to MDE.
You should also setup your Family & Friends list in MDE before you link all of the room reservations.
If you have multiple families, I would recommend at least one MDE account per family unit. If each family is in their own room, you can link the room to that families MDE account. Once the individual family accounts are created and room reservation linked, you can use the room reservation number to link each families MDE account to the other families.