As a Team Founder, I have found a few solutions:
1) Some projects do not allow you to view your own teams member email addresses any more. But, if each team member has allowed the founder to send them an email, then you can do that directly from the Admin page of the Team section on the project website.
2) You can message team members agains via the Admin section...but this only works if the member checks his own membership account (to view the message). Some projects do allow members to recevie a notification that there is a new message in their Inbox (on the project website).
3) Another option is to form a forum somewhere else, that members can join - whether this is your own dedicated bulletin board type forum, or by using a social media account, such as Facebook, Twitter etc.
4) Of course, with GDPR now in place, you may find that keeping personal records of members email addresses, might now be an issue...esp if any other personal data is kept with it, such as real name, and any other contact info. And keeping records for membes who no longer run BOINC isn't a good thing either, as those who've dropped out, have done so as they don't want to be involved anymore. So, maybe better to ditch those old records?
Ultimately, you just need to find an option that works for your active membership and if it is high profile enough, maybe you might attract new members ?
Best of luck
Tim
Founder, UK BOINC Team