As @merefield points out, advice for growing a ‘quiet’ community is pretty much the same as starting a new one, but at least you have the possibility of interacting more with existing users.
First, you need to create engaging content by getting your more motivated regulars and ambassadors to talk to each other, to demonstrate the value of the topic that lies at the core of your community’s identity.
You also want them to model good behaviour, so others can see what they are supposed to do and how they too might get involved.
I’ve found that regular events where you get members together online at the same time, for an AMA type event or Q&A, gives some added vibrancy to the community. Not only do you create good content, but it is easy to follow for others since it will read more like a social chat than a series of pontifications by experts.
Resist the call to post very general ‘social’ content - like “Who will win the World Cup” (unless you are a sports community) because it is something they will already be seeing in many other places they may currently have stronger relationships with. Try to keep things fun and social, but on-topic, at least while establishing THIS community.
When re-engaging previous registrants, don’t take them for granted. Make special effort to communicate to them as personally as possible to explain the reasons and benefits of the new platform. Offer to remove them or to help them get set-up their new profiles. A community emerges not just from shared identity, and engaging content, but from the tools & processes that are used to interact. I do believe Discourse is a wonderful platform for this, but if they are already used to something different, there will be some resistance, so be open to feedback.
Lastly, do think carefully about how you set up the incredibly useful digests. Email is still one of the best ways to get repeat visits - at least until they become properly engaged and make the community a place they monitor actively (such as by putting the app on their mobiles and getting notifications).
Make sure that the interval you select as a default is suitable to your community. There’s no point sending a daily email if new content is only posted weekly, and vice-versa, it is a waste to send a two-weekly email if there will be great content posted every day that might be missed.
Let us know how it goes