You can change a guilds starting roles in the BP_CGGGameStateComponent class defaults in the Starting Roles. These are the roles that guilds start with by default. Here you can set the role name and its starting permissions.
If you change the Owner role name you MUST ALSO CHANGE THE Owner Member Role Name to the same name like how its set to Owner by default.
If you change the Member role name you MUST ALSO CHANGE THE New Member Role Name to the same name like how its set to Member by default.
You can also enable the Use Max Guilds Size, this is how many players can be in a guild.