(note: the response provided by someone else above is not 100% correct, and has possibly been generated by AI.
Power Pages can be configured (often is by default) to use your Azure AD, if you want to allow all your Azure AD users to be able to use Power Pages then you just need to ensure it has been enabled/configured in Power Pages - however, also note that unless your users are obtaining licences from other qualifying products you will also need to purchase authenticated users packs to ensure appropriate licencing.
Assuming you do not already have your users as Contacts in Dataverse
If you want all Azure AD users, then when they first register/sign-in a Contact (portal user) will be automatically created for them (assuming the an Office 365 Global Admin has signed in to the Power Pages site and Accepted on behalf of your organization or they have configured the Power Pages site's app in Azure to auto accept).
If you do not want all your Azure AD users, then you will need to look at a slightly different approach e.g.
- Turn off open registration, and a Create Contact record for each user and then send them an invitation; or
- You may be able to configure the Azure Application for your Power Pages site (one is created for each site), and configure it so that only specific users or groups can access it. (I haven't tried this option myself but believe it is possible).
(if the contact is already in Dataverse, you can turn on the email mapping https://learn.microsoft.com/en-us/power-pages/security/authentication/openid-provider#additional-settings-in-power-pages and when the user first registers it will use their email address to link the Azure AD to the Contact record)