Usually, guest user has to get provisioned in Azure AD tenant for application access. This can either be done via invitation or adding guest user directly or by redeeming an individual invitation.
If there are lots of users that needed to be added as guest in Azure AD then you can make use of feature bulk invite Azure AD B2B users.
https://learn.microsoft.com/en-us/azure/active-directory/external-identities/tutorial-bulk-invite
You can also make use of Self-service sign-up.
When sharing an application with external users, you might not always know in advance who will need access to the application. As an alternative to sending invitations directly to individuals, you can allow external users to sign up for specific applications themselves by enabling self-service sign-up user flow.
You can refer below articles to know about self-service sign-up and to configure the same.
https://learn.microsoft.com/en-us/azure/active-directory/external-identities/self-service-sign-up-overview
https://learn.microsoft.com/en-us/azure/active-directory/external-identities/self-service-sign-up-user-flow#before-you-begin
https://learn.microsoft.com/en-us/azure/active-directory/external-identities/identity-providers
https://learn.microsoft.com/en-us/azure/active-directory/external-identities/direct-federation
https://learn.microsoft.com/en-us/azure/active-directory/manage-apps/manage-self-service-access
Let me know if you have any further questions.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.