There's a range of options depending on exactly what you want to do. Login as an Administrator and follow the menu options below:
OPTIONS > USERS > MANAGE SUBSCRIBERS
Presents a list which you can "see". You can't "download" it as such but select and copy the data on the page and you find it forms a tab-separated list of the fields:
iconname > userame > address > subscription type
which doesn't take much to process in a decent text editor or spreadsheet so you can extract the addresses alone
However, you say "forum" rather than "mail list" so you may mean the various classes of user, in which case try:
OPTIONS > USERS > MANAGE USERS & GROUPS
Presents a set of pages which list the username and address of the members of your various classes of user. The address itself is always enclosed in angle brackets so, again, it should be easy enough to extract using appropriate software tools.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.