Granting access rights to a Wordpress website

Hello everyone, besides an existing website I am also building a phprunner web application. The website can be visited by everyone, but there is an exception, because some pages are only accessible to those who have a login in the web application. Finally I have this question: How can I pass on the access rights, originating from the web application, to the existing website that is developed with Wordpress? All comments and remarks are always welcome, thanks in advance.