I know this is a very old post but could you expand on this.
I tried adding :
<input type="submit" name="do=user_profile_email_add;forum=<%forum_id%>;next_do=forum_view;update=1" value="Subscribe to this Forum" class="submit">
but i just get "No such action 'user_profile_email_add"
I'm assuming it is because there is no action associated but I tried to add one without success.
Any help would be appreciated from anyone as I am new to GForum
Edit Darn it... just realised I didn't see the plugin file hidden away on the left