Team Tactics dossier [WIP it good]

Tactics:
ABANDON SHEEP - Full withdrawal to the next safe point(usually a tower), letting opponents take whatever objective you were at. Applicable to most situations except nexus defense.
The reasoning behind this is to preserve your own life, not just to deny the enemy a kill, but because you will be needed to help defend against the next attack.

Conventional Deathball - All 5 team members rally mid and push towards enemy nexus.
Tanks and bruisers make up the frontline, looking to bait out abilities or engage.
Shoot boyz and squishy support are backline, clearing minions and poking.
Assassins and mobile/burst mages lurk around the sides and jungle for chances to kill a high priority target.

Self sufficient engage: the standard engage, champion strikes target of choice using their own cc or gapclosing ability, and assumes all the risk by relying on their own innate defenses(health, resistance, dodge) to keep them alive. This technique is viable for Assassins and Bruisers, tanks can but typically lack the damage to kill in a timely fashion.

Ally assisted engage: champion initiates an attack they could not safely do alone, relying on an ally's ability or the team followup to make the attack successful.
Example - Fiora tower dives an enemy mage, Shen's ult giving her a large shield to mitigate alpha damage.
Obviously requires a high degree of coordination between teammates.

Tenuki: ABANDON SHEEP times 3. Full withdrawal from a losing lane, letting the lane opponent(s) take all towers in their path. You should have a clear idea of where you'll be going instead and be absolutely sure that there's no hope of coming back due to either the jungler being unavailable to help or a general 'apocalypse' situation where everyone is losing lane.

Scenarios:

The Tower Siege: enemy team numbering 3 or more bearing down on a tower, presumably with the intent to kill.
How to respond if you have:
1 player, others are dead/in other lanes - ABANDON SHEEP. If your range exceeds theirs you may try to poke them as you run, but do not put yourself at risk.

2 players, everyone else is dead - if there are 4 or 5, ABANDON SHEEP.

2 players, others are distant - If both at full health, consider defending based on roles present. If one or both are low, refer to 1 player situation. When the siege is broken, do NOT chase or counter engage unless reinforcements have arrived.

3-5 players - the murkiest situation. standard teamfight.

Shoot boyz - Stay behind tower. Only attack melee champions who have entered tower range. If they are playing too safe focus on clearing minion waves as fast as possible. As usual, be wary of an assassin flank or a tank/bruiser dive.
Support/Tanks - prevent/mitigate poke on more vulnerable teammates, and thwart enemy assassins.
Assassins/Bruisers - Look for chances to counterengage on high priority besiegers.

Reinforcing - If YOU are the one who's away while a tower is under siege, see if you can get there via teleport or transportation ability. If you have time to just walk, always take the safest possible route unless you are sufficiently tanky/slippery and you want to flank.

The Dragon contest, offensive: Either you've spotted it with wards/vision or the whole team is MIA and you suspect they could be doing it.
How to respond if you have (close by): 1 player - if you are the jungler or have a smite-like ability (Cho'Gath, Nunu) you can try to jump in and steal it. Use discretion, as you will probably die even if you succeed. If you are not the jungler don't even think about it, barring extraordinary circumstances.

2 players - Are you the bot lane duo? Just ABANDON SHEEP, and don't push the lane too hard as you'll be in the perfect position for them to come down on you after doing dragon.

3 to 5 - Consider the orientation of the pit entrance to your side. If you're bottom side and contesting dragon, you'll have to either go around or over the pit wall, same goes for if you're top side and contesting Baron/Harold.

The Dragon contest, defensive: You're doing dragon when enemy champion(s) appear in river.
Teamwide decision must be made to either commit to finishing the mob or pull off to engage the enemy. Note that finishing the mob can consist of 1 person continuing to fight the mob while the rest distracts the enemy. If the call is made to commit, the jungler must stay near the mob, or within smite range so they can secure the buff.