Should I put the group email address that already exists?
I'm creating a group for the organization from IAM & Admin. There is a "Group email address" input box, but I don't know what kind of email address they are asking for. Should I put a group email address that already being used? Or is this for the new email address that GCP will create? If so, when and how can I use this new email address?
Let me summarize our discussion at the comment section. Let's start with IAM Overview where at the section Google group you can see:
A Google group is a named collection of Google Accounts and service accounts. Every Google group has a unique email address that's associated with the group. You can find the email address that's associated with a Google group by clicking About on the homepage of any Google group. For more information about Google Groups, see the Google Groups homepage.
Google Groups are a convenient way to apply an access policy to a collection of users. You can grant and change access controls for a whole group at once instead of granting or changing access controls one at a time for individual users or service accounts. You can also easily add members to and remove members from a Google group instead of updating an IAM policy to add or remove users.
More information you can find at the documentation page Managing groups in the Cloud Console, for example instructions how to create, edit or delete a group.
Group email address is a unique identifier in form of the email address, similar concept you can see for a service account such as sa-name@project-id.iam.gserviceaccount.com.