PlayerLadderC

This mod allows you to sit on others players heads using the command /playersit <player>
The difference in this fork is that it uses a command, instead of right-clicking the player thus making it a lot nicer to use, at least in my opinion

You cannot do this on yourself
You cannot do this on multiple players at once (e.g. one person cannot be sitting on 2 people's heads at the same time)
You do not need operator permissions for this command
You must be within 5 blocks of the player you wish to sit on

And no, there is currently no way to kick a player off