A user is not able to add a group to an Override Rule for the Coordinator role in Vault.
The Coordinator role is a single-user lifecycle role.
The Owner and Coordinator lifecycle roles are single-user roles, where only one user can be assigned to these roles at a time. Since groups may contain multiple users, they cannot be assigned to a single-user lifecycle role.
Vault Lifecycle Documentation: Defining Allowed & Default Users for Roles