Add user to a group after the user makes a comment in that group
The answer is ... yes, it is possible ... and easy to do ... with Rules.

If anyone coming across this is new to Rules or needs a refresher I highly recommend Johan Falk's free screencast series. It's excellent. http://nodeone.se/en/learn-the-rules-framework

See the screenshot for how I did it, and also the exported Rule below.

{ "rules_lambeth_comment_join_group" : {
"LABEL" : "lambeth comment join group",
"PLUGIN" : "reaction rule",
"REQUIRES" : [ "rules", "og", "comment" ],
"ON" : [ "comment_insert" ],
"IF" : [
  { "entity_has_field" : { "entity" : [ "comment:node" ], "field" : "og_group_ref" } }
],
"DO" : [
  { "og_subcribe_user" : {
      "user" : [ "comment:author" ],
      "group" : [ "comment:node:og-group-ref:0" ]
    }
  }
]
}
}