Group Claims are not available out-of-the-box with Azure AD B2C.
In standard Azure AD tenants, Group Claim can be returned by configuring it Token Configuration blade of the registered application but in Azure AD B2C you cannot do that because the token issuance is handled via IEF so the group claim must be added as an output claims to the user flow or custom policy.
As of now, there is no option to get the
Group claim in the token issued via standard user flows and a custom policy needs to be used for this purpose.
In order to get the group claim, you need to use Custom Policy that makes the below Graph call via a RESTful Technical Profile to get the user's group membership and return all groups the user belongs to.
Where the value for
obj_id_of_the_user can be provided as an input claim to the RESTful technical profile and this technical profile should be configured to return the
group claim as a string collection, which can then be added to the claims bag to be returned in the token issued to the relying party.
Let me know if you have any further questions on this.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.