in gcloud console iam how can I add roles to groups?

There is no special action to take if you want to grant the roles through the cloud console, as indicated by the help text on the input field:

Simply write the group email, and select the roles you want to grant.

Note that the group must be a Google group or a G Suite group to work (Google needs to know who is a member of the group to grant the right permissions to the accounts).

The detailed steps are on the IAM docs.