They will be able to access computing sandboxes and the course catalog, just like you are. They will not be able to make changes to the team's settings.
No, only you have the ability to add or remove team members. You can however share a join URL that anyone can use to join.
You'll be billed $19 per month per team member. You can add or remove team members at any time.
Currently, the work each teammate does is private to them. They can share a direct URL to their sandbox, however!