There aren't many tables for eight in most of the restaurants so it will be more difficult. That said, start by trying to book as a party of eight because then the restaurant seater will know that you will need a large table.
If a reservation for eight isn't available you can try booking smaller reservations at as close in time as possible (e.g. two tables of four), although the reservations will need to be made in two different accounts. Also, the restaurant will assign you to two tables for four. When you arrive you can ask to be seated together and they will try their best. It may be accomplished easily or you may have to wait longer or it may not be possible and you will be seated at separate tables near each other or on opposite sides of the room.
As an aside, Biergarten in Epcot is communal seating and virtually all the tables are for eight.