motivate them with high goals, involve them intro kingdom strategies (make use of inteligence of every player and even if they are stupid ask them what they feel about this strat or that or this). make them feel important part of the kingdom and of course important part of the utopian community.

when a player feels important part of the utopian community he will never lose interest, on contrary he will involve more to get better himself. sooner or later he will figure out that without team he cant get anything alone, therefore he will involve more to improve the team.
well i'm impressed, you may have some potential after all :p