Before rolling Engage out to members of your church or organization, it is important that, to the best of your ability, you make sure you have current email addresses attached to individuals in your CDM+ database.
When a person creates an Engage account, Engage will ask them for an email address. Engage will then use the email address to see if the person already exists in your database. If so, the person's Engage account will be connected to their existing address, individual, and giving unit records. If Engage does not find a matching email address, though, it will create an account for the person, and it will also create new address, individual, and giving unit records to go with that account.
Doing the up front work to have current email addresses can prevent unnecessary creation of duplicate records.