Cheryl
As far as I know you are invited to book a slot if you have an account with them, but having an account doesn't guarantee an invitation. There is a post on MSE where a lady wrote to them asking for an invitation and was basically told that it was hit and miss whether you get one! (see below)
"Dear Ms XXXXXX,
Thank you for your email regarding the Internet VIP sale.
I am sorry you did not receive an invitation to place a VIP sale order on the Internet.
I can confirm that customers were selected at random to place their order. I am sorry you were not selected this season.
Customers can register their interest in being invited to the Internet sale by logging onto next.co.uk, then selecting "my account". This will display a page showing your registered email address and a selection of 4 email preferences.
To ensure you are considered for selection for future Internet sales, you will need to place a tick in the box of "send me emails of offers and promotions" and "confirm my orders by email".
Although I cannot guarantee you will automatically be selected for our next VIP Internet sale, this will however update our records to show that you wish to be invited for any future Internet events.
Once again, I am sorry for any disappointment this may have caused.
Regards"