MySQL Group Replication multi-primary - add new member
If you want to add a new member to current online Group Replication, You don't need to stop or restart any of nodes.

For doing this just config new member(s3) like current members(s1,s2) but add new IP at the end of these two variables

loose-group_replication_ip_whitelist = "x.x.x.x,y.y.y.y,z.z.z.z"
loose-group_replication_group_seeds = "x.x.x.x:33061,y.y.y.y:33061,z.z.z.z:33061"
loose-group_replication_bootstrap_group=off
loose-group_replication_start_on_boot=off


After that Start new member and execute these

SET SQL_LOG_BIN=0;
CREATE USER rpl_user@'%';
GRANT REPLICATION SLAVE ON *.* TO rpl_user@'%' IDENTIFIED BY 'password';
FLUSH PRIVILEGES;
SET SQL_LOG_BIN=1;
CHANGE MASTER TO MASTER_USER='rpl_user', MASTER_PASSWORD='password' FOR CHANNEL 'group_replication_recovery';


Then Install group_replication and start group_replication

After the new member joined the group open my.cnf of s1,s2 server and update loose-group_replication_ip_whitelist and loose-group_replication_group_seeds to this:

loose-group_replication_ip_whitelist = "x.x.x.x,y.y.y.y,z.z.z.z"
loose-group_replication_group_seeds = "x.x.x.x:33061,y.y.y.y:33061,z.z.z.z:33061"


But you Don't need to restart these nodes(s1,2) just update my.cnf