How to force a group to sync with Google?

[Gustavo Juarez]

We have a group that syncs users from Google and we have set some permissions to that group. We have a new hire and it has not been synced and I can't add to the group because the button "add" has been disabled and it says "you can't add users to this group because it is synced from an external directory" is there any way to force the sync or how can I add the user manually?