If a user is updated through the normal SSO login process the update will not occur until they logout and log back in again. The code example above is the easiest way to approach the problem, but it’s probably not the best way to manage group membership.
The WP Discourse plugin has a couple of helper functions that are enabled when WordPress functions as the SSO provider site for Discourse. These functions allow you to update group memberships without requiring the user to logout of Discourse. Those functions are in the topic that Richard linked to: Manage group membership in Discourse with WP Discourse SSO.
The post that Richard linked to gives some details about the two different approaches. I’m going to add those details to the howto topic next week. For now it would be good to read through the post.