Minecraft – How to set new players into a team

The command to check if anyone on team red has killed an ocelot is

/testfor @a[score_leader_stat=1, team=red]

Note that there's an underscore between score and leader_stat, not an equal sign! Get the output of that command block and hook it into two other command blocks with

/tp @a[team=red] x y z
/tellraw @a {text:"Red Team has slain the Ocelot!", color:red}

In a 1.8 snapshot, you could also use

/title @a title {text:"Red Team has slain the Ocelot!", color:red}

to get a large text display. You will need a set of such command blocks for every team and every possible leader. Note that if red team's leader is an Ocelot as well, they will also win the game by killing their own leader.

Information taken from the wiki:
http://minecraft.gamepedia.com/Scoreboard#Teams

To teleport players in specific teams, simply add the team=[name] filter onto the @p command, resulting in:

/tp @a[r=10,team=red] 100 100 -100

Just note that everything is CaSe SeNsItIvE (Case sensitive). It means that 'red' is different to 'Red'. Replace the word 'red' with the name of the team.

To set make team, use
/scoreboard teams add [name] [display name... (This can be more than one word)]

And to add a player to the team, use
/scoreboard teams join [team] [player]