URL: https://www.zabbix.com/documentation/3.0/manual/config/users_and_usergroups/usergroup

The way to access the user groups are changed in Zabbix 3.

I sugest change this:
"Configuration

To configure a user group:

Go to Administration ? Users
Select User groups from the dropdown to the right
Click on Create user group (or on the group name to edit an existing group)
Edit group attributes in the form"

to this
"Configuration

To configure a user group:

Go to Administration ? User groups
Click on Create user group (or on the group name to edit an existing group)
Edit group attributes in the form"