You can normally book yourself into a slot through the online booking system, however in some cases clubs won't show tee time availability if a member has booked already, even if there is space available.
If the full member has already booked a slot
You may not be able to book this slot online, due to limitations with some clubs' tee sheet systems. In this case, it's best to contact the club directly. They'll be able to book you in on the slot, and ensure that your points balance is updated.
If the slot is free
If the slot you'd like to book is free, you should be able to book it online. Once you've done this, ask the full member to contact the club so that they can be added onto the tee time. If you intend to play a lot of rounds with friends, it's always easier for a PlayMoreGolf member to make the booking online first, and then for the full member to be added to it by the club.