If your CA firm or your employer uses ReturnDone, they invite you to a portal of your own. There is nothing to sign up for: open the invitation, set your password, and log in.
No. Your CA firm pays for ReturnDone; your login is included at no cost to you, and it never counts towards their seats.
Only you and the people working on your file. Clients and employees see only their own records, every file your firm opens is logged, and files are encrypted at rest.
Not this page. If you want a CA to file your income-tax return for you, see how it works for individuals. If you run a CA practice, see ReturnDone for firms.