The goal is to set up, maintain and link several local underground mail networks. There are other similar groups, some currently active, others transient and short-lived. For example Sf0.
Groups use various different techniques to maintain their network. For example, a group near Brighton, UK, appropriates Bookcrossing release spots, to send and receive mail. See Potsmaster.
To participate, first see if there is a suitable group already active locally. If you cannot find a local group, then start your own. Please post details to S.T.A.M.P. and also to @Plot49 with #Trystero on Twitter. W.A.S.T.E.