Secure user account activation after user registration

I have a web app (b2b) in which users access to it through user name and password. Now I am looking for a system so that after user registration for the first time (user data provided to get user name and password to get the permission to enter to our web app), we can ?activate? their account in a secure way. Because we want to assure that the user who has registered into our website is the ?real? user who will use it (this means, prevent users providing data from other companies and enter to our web app with this false identity).
Thank you
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