Make sure you have the correct email addresses that new team members will use for their accounts.
People you invite can convert their existing Dropbox account to join the team.
If you're already a Dropbox Pro subscriber and you purchase or join a Dropbox for Business account, the amount of credit remaining in your Pro subscription will be refunded to you.
This article refers to a feature unique to Dropbox for Business. Dropbox for Business is aimed at projects, groups, and other organizations that want to share the same Dropbox storage quota with all of its members. If you're interested in learning more or want to sign up, visit the Dropbox for Business webpage.