ok - the answer seems to lie in admin/gforum/user.pm - in the subroutine 'signup'

one needs to create a language variable similar to this:

USER_EMAIL_EXISTS

Message:
This e-mail address ('%s') already exists for another account. Rather than creating a new account, please request a temporary password under your old account. To do this, enter your old user name and click on "Lost my password." If you entered this e-mail account by error, please enter the correct address and try again.

A coding guru could figure out how to make it say:

"This e-mail address ('%s') is currently being used by <%user_name%> and can not be used for any other accounts. If this is your correct e-mail address - please do not create a new account. Instead, we ask that you request a temporary password under your <%user_name%> account. To do this, click on "Lost my password" and enter <%user_name%>. A new password will be sent to the e-mail address listed on the account. If you entered this e-mail account by error, please enter the correct address and try again."

That is far as I can get - I simply don't know how to code it.