Can I use the same email address in multiple accounts?
No, not directly, you have to use a different email address in each of your Scrumwise accounts.
But, you can use the following trick to create as many email addresses as you need that all point to your real email address: Add a plus sign ("+") and any combination of words or numbers just before the '@' character in your email address. For example, if your email address is firstname.lastname@example.org, and you would like to use that email address in three different Scrumwise accounts, you could use the email addresses email@example.com, firstname.lastname@example.org, and email@example.com.
Scrumwise will remove the part after the plus sign before sending mail to you, so all mail will be sent to your real email address (for example, firstname.lastname@example.org). But when you log into Scrumwise, you can use the part after the plus sign to select which account to log into. This works regardless of whether your own mail server supports the plus notation, since Scrumwise removes the plus part before sending mails to you.
If you use this trick when creating an account for somebody else, make sure to tell the person which email address you've used, since the person needs to use that email address to log into the account you create.