There are a few ways you can contact other Portishead fans to talk about the band and ask questions:

You can join the Portishead email mailing list by going to http://www.Mailing-List.net/list/portishead/portis.htm

There are some message boards at the official page: a general message board, one for Geoff, and one for Adrian, where the band members themselves occasionally post messages..

And there is a newsgroup: alt.music.portishead.