Note that the usrquota and grpquota options don't necessarily enable quotas on a filesystem. You can make sure quotas are enabled using the quotaon command:

# quotaon /usr/users

There is a corresponding quotaoff command should you desire to disable quotas in the future:

# quotaoff /usr/users

But for the moment, if you're trying some of the examples in this tutorial, be sure to have quotas enabled.