How can I create a team in itch.io?
and How can I invite people to join my "team"?
I dont think that is possible. You would have to work outside of itch.io with other people and then turn in your project as one. I could be wrong though.
You can use github to collaborate with teammates.
Yes, I see no such option, 10x.
It will be nice for teams to have that option for game-jams.