I have three suggestions
First: Some type of an award for being active on this website. For example, as you probably know, in Days of Wonder, MM 44 a player can receive a medal or get promoted up a rank for various accomplishments in battles or other reasons. Could this be done on this site also? For example, posting in the forum, submitting scenarios, links, related articles and so on.
Second: Recruiting. This is linked to the suggestion above. Again at the MM 44 you can recruit others to MM 44. Here the link for that recruiting page
www.daysofwonder.com/memoir44-online/en/recruit/
The page is titled Band of Brothers. There it reads “You may use this page to help you invite friends and make them discover Memoir '44 Online. Two Achievements can be awarded: "Band of Brothers" if at least 5 people responded to your invitation successfully and "Band of Brothers Instructor" if 3 of them graduated to 2nd Lieutenant or better.” So while playing BC at Game Table Online, I would ask the other player if they would be interested in being invited to MM 44. If they were I would go to the game roster and look them up, and if there email was listed I would send them an invitation. If their email was not shown I would send them a private message asking them for their email. Having their email address I would send them a message Title: Join my Band of Brothers on Memoir '44 Online
From: xxxxxxxxxxxs@xxx.com
To: xxxxxxxxxxxs@xxx.com
There was a link there for them to join, and I would get credit for them signing up.
Third: Chat. Would there be interest in chatting with others who were online? One could to the user list and ask if that person would be interested in chatting.