Zabbix 1.8.0 - Creating a New User and Adding them to an existing group will wipe out the other users in that same group.
Go to "Administration" -> "Users" -> "Create User"
Fill in User Info and add the new user to a group that has users already in it.
Save
Go back and check the "User Group" that you add the new user into and that user will be the only user in the group.
Go to "Administration" -> "Users" -> "Create User"
Fill in User Info and add the new user to a group that has users already in it.
Save
Go back and check the "User Group" that you add the new user into and that user will be the only user in the group.