I created an openid provider. Keycloak/RHBK 26.0    

Keycloak with user which is member of a group
IDP contains
      claims: 
        ...
        groups:
        - groups    

    1. create RHBK
    -  create realm
    -  create user
    -  create group
    -  add user to group
    -  create client
    2. create secret for client
    3. create IDP
    - name: rhbk
      openID:        
        claims:
          email:
            - email
          name:
            - name
          preferredUsername:
            - preferred_username
          groups:
            - groups
        clientID: ocp
        clientSecret:
          name: rhbk-secret
        issuer: 'https://keycloak-rhbk.apps.<openshift-domain>/realms/ocp-realm'
      type: OpenID
    4. login with IDP rhbk and user   

NS: openshift-authentication Pod: oauth-openshift-f7cbdc5b9-8gb7t
E0212 06:41:33.325983 1 errorpage.go:28] AuthenticationError: Group.user.openshift.io "/rhbk-idp" is invalid: metadata.name: Invalid value: "/rhbk-idp": may not contain '/'    

no error, login should work    

when the claim for group is removed, login works, but group does not exist    

Group is documented at
https://docs.openshift.com/container-platform/4.17/authentication/identity_providers/configuring-oidc-identity-provider.html#identity-provider-oidc-CR_configuring-oidc-identity-provider