How to share iCloud+ with family member on same account?

If you are using the same Apple Account (formerly Apple ID) then you have only one account and should set up a second Apple Account for one of the two people. The one account will already be using the same iCloud+ space on each device signed into iCloud with that account's credentials.

To share an iCloud+ subscription with other Apple Accounts you need to have a Family Sharing group, and in joining the group choose to use this shared iCloud+ plan.

If you are sharing one account then the iCloud storage is already being shared. It is like having a single room in a house and sharing the house with another person. You are both in the same room with no divisions.

You only need to proactively share if you each have your own Apple Account, as you should. Make sure that each family member has a unique Apple Account - Apple Support

- "If you want to share purchases or subscriptions with family, use Family Sharing. Each person should use their own Apple ID."

Then you set up Family Sharing and share a family level subscription. Read this article to learn how to share iCloud+ with your family --> Share iCloud+ with your family - Apple Support