If there are 0 open slots then the map has reached it's player limit (is full) and new players can't join that map. For example if a map allows 100 players to play it and 100 players join that map then it becomes full.

As the game admin you can kick inactive players to open slots for new people to join that map BUT I'm not sure if players can join a game that's on day 9-10 and even if they can you will probably have to ask your friends to join that map as most people first, won't be able to find it and second, probably won't want to join as countries that are played should have progressed a lot in the past days.

As h4zel said, you can only join a map until the max join date. With that being the case, the game admin function stops working after that point because you would only open up a bunch of slots without them being able to be refilled after.

Inactive players are not open slots. Only completely AI nations are open slots.

What you say makes it sound like inactive players cannot be removed at all, which is not the case.

Game admins can only remove players in player created rounds.

If you are the game host/admin in player created games or 22 player and smaller non-tutorial rounds you have the ability to clear inactive players and open the slots for new players to join.

This function has not yet been implemented on mobile, so is only available in desktop mode. Click on your name/flag in the upper corner of the map and you will find the admin panel. You will have only one button that clears all inactive players.

You have a duty to use this power responsibly and inviting your friends to stack the game against your enemies in this case is strictly prohibited.

All games ( except for private password protected games) have a join limit that prevents players from joining after a certain point, most maps this is 10 days. After this time even with vacant slots the game will show as “too late to join” and new players will not be able to join.

The button to kick inactive players is not enabled in larger system generated maps and event games.