I believe there are two shared lounges at FCO, as a WC member I have used the one on the level above the KLM flights to Amsterdam. If memory serves it is on the B concourse. It is a very nice lounge. They offer snacks, mini-sandwiches, drinks, reading material and a shower.

I am sure that Skyteam Elite Plus will work at some of the other lounges, but WC membership will work at this one.