Once you have created a new match, you can assign players to be captains for the match.
From the Availability matches list page, click the Captains button on the relevant match:
On the Captains page, you can select up to 9 players as captains:
On saving this page, the system will send emails out the selected players:
This is all the manager needs to do for now, until the captains invite players and any other action is needed.