I think I have solved it.
It appears that there is a difference between Group Replication and Innodb Cluster that perhaps needs to be made clearer in the documentation.
Anyway, I ran:
mysqlrouter --bootstrap root@[::1]:3306
and got:
Error: Expected MySQL Server '::1:3306' to contain the metadata of MySQL InnoDB Cluster, but the schema does not exist.
Checking version of the metadata schema failed with: Error executing MySQL query: Unknown database 'mysql_innodb_cluster_metadata' (1049)
This was the first clue that something was missing. Then I eventually found the following:
https://dev.mysql.com/doc/refman/8.0/en/mysql-innodb-cluster-from-group-replication.html
This tells you that you need to "convert" group replication to an Innodb Cluster by running:
var cluster = dba.createCluster('prodCluster', {adoptFromGR: true});
This tells you that:
A new InnoDB cluster will be created based on the existing replication group on instance 'localhost:3306'.
After that I tried bootstrapping mysqlrouter, and it worked.