It exists on couchsurfing.org, even if now it works rather worse than two years ago.
As here on the majority of groups, messages are not frequent, when there are some, it is not a big problem if daily oy weekly digest are not yet provided.
But it would be nice to make that working later.
Another problem witch also concern instant message notification, is that if à first version of a message is sent folowed by some corrections, that will do 2 messages sent to recipients with often few differences between them. It would be beter to delay (of one hour) new messages notification. So if the message is changed only the last version would be sent by email.
A little the same if the message is written several languages. Not worth to send everybody every versions in all languages.
If daily or weekly digest are done, it would be necessay to get the real content of messages. Not links asking to connect to bewelcome (like couchsurfing.org does now for it's groups).
According to the other question about the choice of a forum software : I think a quality of bewelcome.org website comparing to the two others websites about hosting travellers is to permit the write the same informations on several languages. I don't think PHPbb offerts this fonctionnality and not sure either PHPbb allows daily or weekly digest by emails.
Bewelcome forum software works (with some bugs to fix like if the messages starting a discussion is edited, the discussion will leave its original group). But it does not seem usefull to change the forum software, this one has qualities that I did not find somewhere else.