You need to
-- write an e-mail message (subject line and contents
do not matter) to pgsphere+...@googlegroups.com (click to show):
pgsphere [PLUS] subscribe [AT] googlegroups [DOT] com
-- shortly after that, you will get an e-mail message
such as "Join request for pgSphere users and developers ..."
Just reply to that e-mail without adding anything.
That should be all. The URL provided in this "join request" message
will, unfortunately, only subscribe a currently logged-in Google account
to the mailing list.
You can also write to mnullmei [AT] ari [DOT] uni-heidelberg [DOT] de
to request a subscription in case the automatic method described above
might not work.