URL: https://www.zabbix.com/documentation/3.0/manual/quickstart/login
In this page is presented, step by step , how create a new user.
And have a reference to a group called 'Network administrators' and this one is not a default group. I know this one is only a example but I think its a good way if the creation of this group is a part of manual (maybe in https://www.zabbix.com/documentation/3.0/manual/config/users_and_usergroups/usergroup).

In the user group manual page have a example about how create a user group, maybe add in this page a example for create 'Network administrators' group or change the change the quickstart user creation for use 'MySQL Administrators'.

In both case I sugest to add a reference in (https://www.zabbix.com/documentation/3.0/manual/quickstart/login) to "https://www.zabbix.com/documentation/3.0/manual/config/users_and_usergroups/usergroup" for user know how create the group to avoid mistakes.