Essentially they will need to activate their account in order to get themselves a password and be able to log in. This can be accomplished by sending customer invites. Since you have 2700 customers I would suggest using a third-party app such as Bulk Account Invite Sender that will allow you to send out these customer invites all at once in bulk.
Let me know if this helps. If there is anything else I can help you with, please let me know.