Minecraft add player to team based on scoreboard score

The problem is that you can't target someone without a score using a score selector. So the solution, not surprisingly, is to give everyone a score. You don't have to change anyone's score value either; instead, just "increment" everyone's score by 0, in your case, with the following command:

/scoreboard players add @a hasJoined 0

Better yet, throw that command in a command block that is triggered off a 20Hz clock, and you're guaranteed that everyone always has a score.

The way this works is that everyone that already has a score in the hasJoined objective won't have their score changed, but everyone else will have their hasJoined score set to 0.

So basically, you have a command that makes 2 scores identical and a score you want to subtract:

/scoreboard players operations (player1) (objectiveA) = (player2) (objectiveB)

(note player one and player two don't have to be different)

Next you would want a command that you subtract from (objectiveB):

/scoreboard player operations (player2) (ObjectiveB) -= (player2) (objectiveC)

Then you have a player selector in the next command. For instance:

/effect @p[score_Output_min=1] minecraft:slowness (time) (amplifier)

and that will show it is working right if you do the same general outline of what I did. Hope this helps. Just heads up this gets complicated real quick and it took me forever to figure out what i did wrong. I hope this helps you understand and recreate the idea if you need it.