Assuming this "Elite Guard" event is the one with raid bosses...
Hold on to your wild cards, at least for the moment. If Slayer Cards are introduced before the event, you're going to want to have those leveled up. If Slayer Cards don't show up or you don't feel like shelling out the money to buy them, I would level up Moe cards to about level 50-60 and then save any other wild cards for Sexy cards.
Friends come into play when you/they encounter a raid boss. As you're spinning the slots, you'll occasionally hit three black icons with female silhouettes on them. When you get these, you'll encounter an enemy much like in the story mode. They'll start pretty weak (1k-10k HP) but it won't take long for them to get much more powerful. If you hit one but don't knock her out in one hit, you have the option to invite up to 20 friends to join in and help you knock her out.
All participants in these fights get items (consumables, WCs, coins) and event points based on how much damage they did (with bonuses like MVP and final hit).
Top scorers for the event get SR cards that are almost never made available again.