The Marketo Program to SFDC Campaign sync relies on the general bi-direction sync with SFDC to do it's updates - this is part of what makes it so efficient with API calls compared to using flow steps like Add to SFDC Campaign or Change Status in SFDC Campaign. However, because of this it can take some time for the updates to occur, depending on the size of your lead database, how busy your sync is, and whether or not there's a backlog. If the members don't show up after an hour or so, I recommend checking in Admin > Salesforce to see if your sync is failing (the last attempted sync time will show up in red in the top-right if it failed).
If you are using Marketo Landing Pages, you will need to use "change program status = Webinar > Registered" to move marketo landing page registrants to GoToWebinar. Hope that helps! - Jeff