Allowing external user access; External account setup

I have THREE questions:

1)  I would like to be able to allow clients (external users) to access folders on my site.  This access would be used to upload files and to work in Word and Excel files (read/write).

Is it possible to grant external users these permissions?

2)  In order for an external user to access the account do they HAVE to set up a Microsoft account?

3)  The process of setting up an account appears to require that the person provide a birthdate and gender...is there a way to provide access or set up an account without providing this personal info?