When new members join, even if they were invited to an invite only network, how about a welcome message you can auto send (and edit) so that they may feel more obliged to share things on your website? Also here is another, a reminder to log in and check their mail on site or for site participation?
Like it's been a week and a half and "X" has not posted anything, a emailed reminder to remind them to post or check in (maybe with an actual check in box) could be mailed.
It could also go by membership roles (free or standard) etc.