I've been playing LoL lately with my four friends and we have done great success with this team. We have tried many other kind of teams before but this is by far the best we've tried and we have just destroyed the enemy team. We have played something about 50 games together, and when we discovered the true power of this team we played like 13 games in a row and won 12 of them easily and the fight we lost was an even fight but in the end we just couldn't work out in team fights so good (i don't know why)... Just so you know we played in 30-level series so you won't start whining about if we played on under 10-level series and all the enemies were complete noobs.

This team is based on massive AoE damage and that's why the members of the team should communicate well for maximising damage output by using the best damage output abilities in the best possible moment (we had a skype always on for better co-op playing).

is the champion I always played. Galio is supportive tank that can deal huge loads of AP damage into enemies especially when he has plenty of magic resistance. Galio is at his best when the enemy team is based more on magical damage instead of physical damage. Galio's ultimate Idol of Durand is the initiation ability in team fights. It taunts enemies so all your team mates can do their own tricks while you are being focused. Galio should have Clarity and Flash chosen so as sumoner spells. Clarity for supporting other members and Flash+ultimate combo for maximizing damage and taunt for as many enemy champions as possible. Idol of Durand should be used when there is 3 or more champions inside ultimate range.
Galio's item build can vary very much depending on enemy team. Aegis of Legion is a great all-around item because it gives tanky stats for team mates nearby. Other items are completely dependent on enemy team. Good choices are of course Banshee's Veil, Force of Nature, Warmog's Armor, Guardian Angel, Thornmail and Sunfire Aegis.
Wards are always needed and in this team Galio should be the one who buys wards constantly.

is a melee dps bruiser champion. He should hold Ignite and Exhaust.
XIn should be building AD items (what a surprise...) and a Banshee's Veil for helping with CC.

is a hybrid dps champion. Her ultimate Death Lotus provides huge damage in team fights.
For Katarina's items I suggest going for 2x Hextech Gunblade (even tho the unique actives cannot be used twice), then Rylai's Crystal Scepter and then Rabadon's Deathcap.

is a ranged AD dps champion. Her ultimate Bullet Time is an AoE ability that fires tons of damage in a cone in front of her.
I don't know her summoner spells or item builds very well...

Maokai is an AP/tank champion but in this team it should be played as a mage and not as a tank because Galio does tanking here. Maokai's Sapling Toss can reveal vision behind terrain and if he see's an enemy, he can travel through terrain with his Twisted Advance. This makes Maokai real good chaser when enemy team escapes in low health. Maokai's ultimate Vengeful Maelstrom is a great bonus for this team. It reduces all damage taken on your team mates by 20% (doesn't work on tower damage). Because this is AoE team it is even bigger bonus because all team mates are inside the battle except Miss Fortune who can be use her ultimate from far away. Maokai should be armed by Teleport and Fortify.
For items Maokai should go for AP items.

Laning should always be done this way:

Mid:

is one of the best mid champions and should be played in mid where she is at her best. Katarina should be ganking other lanes constantly.

Lane #1

:
and on one lane. The trick in this is that when both Galio and Zin hit level 6, they can destroy their lane enemies pretty easily with both ultimates done at same time. If enemy team has Shen this can be a little risky but still you can chase low health enemies down if you are doing the ultimate combo elsewhere than under the enemy tower.
This lane should be played pretty passively because both champs at this lane are melee and don't have that much dps before level 6.

Lane #2

:
and on that lane where are the two jungle golem creeps. With Maokai's saplings the golems are easily nuked down and more gold and expirience are flowing in. This lane has some great harassing power and that's why this lane should be pretty aggressive and make enemy champions lose their farm by standing way behind minions.

Because every champion has an ultimate that deals AoE damage the team fights are very entertaining to watch.

Fights should start like so that all 5 team members are gathered in one place near oppositing group. The tactic is just to be patient and let enemies come frustrated and start to harass too aggressively. When this happens Galio should blink in with his Flash, use Bulwark on yourself and cast Idol of Durand to taunt enemies. Galio must not interrupt the spell cast for immediate damage. Just taunt the enemies the full time and let your team mates get more time to come in.
When Galio is taunting Kata, Xin Zhao and Maokai jump in. Maokai casts ultimate to reduce the damage taken after Galio's ultimate blows up. At this time Xin and Kata cast their ultimates while Fortune comes to good range and does her ultimate. If the enemy team survives from this (they probably wont..) just chase them down and push towers while they're dead and win the match.

I guarantee there is nothing more hilarious to watch than watching this team nuke all 5 enemies down in 3 seconds.

I hope you understood this (my English isn't very good). Don't judge me for not knowing Miss Fortune or Maokai tactics really well, because I haven't bought those champions. I just know Galio, Kata and Xin stuff pretty well.
I hope you and your friends enjoy playing and owning with this team and watching enemies to pee in their pants and whine about how OP all these 5 champions are...

Check out my Zilean build as well: http://www.mobafire.com/league-of-legends/build/zilean-the-supportive-specialist-67581