Hello Anchal Jain,
Thank you for posting this question in the Microsoft Q&A Community.
I understand that you would like to know if a user that goes through the Entra ID B2B user flow journey can be assigned directly to the application added to the user flow or through a Group.
Please be informed that users that go through the Entra ID B2B userflow will be automatically assigned to the application you added to the flow. However, it is not possible to assign this user automatically through a Group if they are signing up via userflow.
All you need do is add this application to the userflow and allow the user to register via that application.
Follow this link to get more information about Entra ID B2B self-service sign up
https://learn.microsoft.com/en-us/entra/external-id/self-service-sign-up-user-flow
Let me know if further assistance is needed.
Babafemi