I can't confirm about bringing guests into the Epic Club, other than I recall reading that you can do so although I think there is an up charge for their meals.
The difference between posh and spice is that Spice is available to everyone, subject to the fact that it is adults only most of the time. Posh, as well as being adults only is subject to a charge.
I believe, and someone will correct me if I'm wrong, that access to Posh comes with a suite on the Epic (it doesn't on the Breakaway where there is a separate sun deck for Suites), and there are a limited number of passes sold to everyone else.
I'd imagine Posh will be your best bet for hanging out with friends as long as they are happy paying the extra and they can get the pass.
Personally, I've never considered paying the extra for Posh on the Epic, not just because we have our son with us, but also because we have never had any trouble finding places to sit. Others seem to have different experiences though, even on the same ship at the same time.
Norwegian Dream - June 2003 12 Night Baltics (Oceanview)
Norwegian Epic - June 2012 7 Night Western Med (Mini Suite)
Norwegian Breakaway - May 2013 7 Night Transatlantic (Oceanview)
Norwegian Epic - June 2013 7 Night Western Med (Mini Suite)
Norwegian Pearl - August 2013 7 Night Alaska (Owners Suite)
Norwegian Getaway - January 2014 1 night CTN from Southampton (Balcony)
Norwegian Spirit - April 2014 9 Night Canary Islands (Balcony)
Norwegian Epic - May 2014 7 Night Western Med (Mini Suite)
Norwegian Jade - Aug 2014 14 Night B2B Greece & Adriatic (Aft Mini Suite)
Norwegian Getaway - Apr 2015 - 7 Night Caribbean (M6 Mini Suite)
Norwegian Spirit - Aug 2015 - 12 Night Grand Med (Balcony)
Norwegian Escape - Oct 2015 - 3 Night Hamburg-Southampton (Angled Balcony)
Norwegian Escape - Oct 2015 - 1 Night CTN from Southampton (Balcony)
Norwegian Escape - Oct-Nov 2015 - 10 Night Transatlantic (B6 Large Balcony)
Norwegian Epic - May 2016 - 7 Night Western Med (Angled Balcony)
Norwegian Star - Aug 2016 - 9 Night Baltics (Balcony)
Norwegian Spirit - Apr 2017 - 11 Night magical mystery cruise (Balcony)
Norwegian Jade - Aug 2017 - 15 Night Iceland (Aft Mini Suite)
Norwegian Epic - Oct-Nov 2017 - 7 Night Rome to Barcelona (Balcony) - Will probably cancel
Norwegian Spirit - Mar 2018 - 10 Night Canary Islands (Balcony)
In the planning: Summer 2018 - Alaska and Hawaii.