There are two roles in a Shared Pro+ account:
An owner is a person that owns the account. He is the one who is paying for the subscription and has control over the members of the workspace to be invited. The owner can share designs with any member within his shared account.
A member is an invited person to a Shared Pro+ account. He can benefit from the Pro+ features like the owner and share designs with the owner and other members of the shared account. However, a member cannot invite others to the Shared Pro+ account as only owners can do this.
It is important to note that whether you are an owner or a member, you can only share designs that are yours and not other people's designs.