Minecraft – How to assign players in MC to a scoreboard team automatically when they join

This is what Command Blocks are for: they bridge the gap between in-game events and console commands. You program one with the desired console command, and hook it up to any sort of redstone trigger. So for example, if you put one next to your spawn point with a button attached, you can give yourself a pickaxe when you respawn there or whenever you need one.

There's a good tutorial with some examples on the minecraft wiki.

I did something like this once. I had a command block on a timer that would look for players not on a team and then teleport them to a room. Any player in that room would be assigned to either red or blue team, using a t flip flop to determine what team to put them on. The flip flop would be switched for every person assigned, so it would go red blue red blue and so on. As soon as they were assigned I'd teleport them out to the team base with another command block.

This was a LONG time ago and I can't remember any more details than that, and there might be more efficient ways to do it now. I just wish mojang would include a scripting language to command blocks because trying to tie the programming into redstone and everything is way harder than it should be and variables can't be moved between blocks. That's why I had to teleport players to a separate room to assign stuff to them.