Go to the match to add the rest of the players. You can do it from the "Matches" section

Once there, open the reservation and click on "edit"

Add the palyers clicking on the slot slots

An alternative option is to share the match link with the players so they can join themselves. In the match view you share the game with them by clicking on the 'Share' icon that you will find at the top of the screen.

Choose the instant messaging app in which you want to share it (WhatsApp, Facebook Messenger, etc.) and have each of them click on the link and add to the game by clicking on the remaining free slots.