Do you currently have an Behance account with the same URL?
If so, you’ll want to change the username on that account to free it up for your Team.
To do this, log into your existing account and go to Account Settings (www.behance.net/account/settings). Under Change URL, put in a new username--it’ll be best if it corresponds to an admin associated with the Team account, so you may want to put your own name here.