If a colleague already has an account under another organization, then it is still  possible to create another account for this colleague. The email address can be the same in multiple accounts, but a username cannot be used twice. If the email address is already used as a username, you have two options to create an account.

You or your colleague can change the existing Username.
Go to Organization in the right menu.
Click on the Colleagues tab.
Search for the relevant colleague in the list or use the search bar and click on Change of the colleague's profile.
Change the username and click Save.

You can import a colleague using an Excel import. Fill-out the basics information of your colleagues; First Name, Last Name, Phone Number & Email and import them all at once. The relevant colleagues immediately receive an email to activate their profile and set a password.

Attached, you will find the Excel file for importing profiles.