At any time (apart from during a race) you can choose to join another rider anywhere in the virtual world. Simply select the “Join” action from the action bar at the bottom of the screen, and then select to join any rider shown.
By default, this action will show all the Pacebots and everyone that you follow. To add more riders to the list, just select to follow them from the profile menu.
Note that there is a known issue with Strava such that it can only draw a ride as one continuous line. This means that if you join another rider in another part of the virtual world when viewed in Strava the route line will incorrectly look like you have traveled between the point when you initiated the join and the point where the other rider is. The FIT file produced by TrainingPeaks Virtual does not include this extra distance, and your ride is shown correctly in two parts (or more, if you do multiple joins!) when viewed in TrainingPeaks Virtual and other 3rd party FIT file viewers.