It is first necessary to ensure that the Group Replication plugin is installed on server s1. If you used plugin_load_add='group_replication.so' in the option file then the Group Replication plugin is already installed, and you can proceed to the next step. Otherwise, you must install the plugin manually; to do this, connect to the server using the mysql client, and issue the SQL statement shown here:

mysql> INSTALL PLUGIN group_replication SONAME 'group_replication.so';

To check that the plugin was installed successfully, issue SHOW PLUGINS; and check the output. It should show something like this:

mysql> SHOW PLUGINS;
+----------------------------+----------+--------------------+----------------------+-------------+
| Name                       | Status   | Type               | Library              | License     |
+----------------------------+----------+--------------------+----------------------+-------------+
| binlog                     | ACTIVE   | STORAGE ENGINE     | NULL                 | PROPRIETARY |

(...)

| group_replication          | ACTIVE   | GROUP REPLICATION  | group_replication.so | PROPRIETARY |
+----------------------------+----------+--------------------+----------------------+-------------+