You cannot shoot into close combat, so if your guardians are in close-combat with the marines you are not allowed to shoot at them under normal circumstances. If you are able to (say, shooting at non-engaged marines) then five Eldar shooting, ten shoots, five hitting and 2 wounding should result in a kill, making the combat a bit easier for the squad involved. They cannot shoot though, as they are involved in the combat.
If you care to elaborate on your question I am sure we can come up with a better answer for you, since your question is a bit weirdly forumlated.