I'll be honest with you. The chance of "boosting" activity via email is low. You will likely see people returning if you send one, so I do advise it, but activity (by which I am presuming you mean on-site participation) will only go up if the site content is engaging to the user.
If they have this reputation already, then either you need to tailor your content to rival their originating site (which will be difficult because you are fighting against familiarity), or you need to create something so dissimilar so they dont feel like theyre doing the same thing in two places. Otherwise I would see about trying to recruit other members all together.
Anyway, the most successful emails -- and i say this both from a personal experience and from a professional point of view -- are:
1. short, direct, and to the point
2. have some easy to understand call to action
3. forces a site visit generally using the call to action
4. gives the user something interesting to consider
Now unfortunately PB forces you to send plaintext emails so you cant design anything that would be immediately eye-catching, but you could do something like this (this is formatted with your forum genre in mind):
Hey everyone!
[Some tidbit about the benefits of mindfulness and meditation here, tying in to an understanding that members may have busy lives AND COULD BENEFIT from taking a step back and engaging in some community interaction]. Come join us in a discussion about your favourite mindfulness and meditation instructors, techniques, and methods.
[put the url to the site here]
See you soon!
[site name] Staff
I would advise AGAINST sending emails talking about inactivity and asking members to pleaaaaaaase login, etc. The most I would do to that effect would be something like a warning email, along the lines of, "As a reminder, accounts without any activity in [time period] are subject to deletion." (I think these work best when tacked on to the end of a positive call to action email tho).
