So, after I have been disconnected from matches mid-game multiple times and some discussions with my mates we have come to a conclusion that you should be able to join games mid-match. A good example for the connection one would be League of Legends; when you get disconnected or you leave you have an option to reconnect. To join a friend mid-match you would go to your friends list and just 'join friend'. This is in most games with multiplayer and there is no reason why it should not be For Honor.