Group Defend
You can send forces to help your allies and buddies defend their planet, moon, or Hephaestus Class Attack Platform.
On the Galaxy page look for the group defend icon 
on the right side of the screen under the Actions column.
Selecting that will pull up a "Choose Ships for Fleet" dialog in which you can select the size of the force you wish to send as aid.
You can also select how long that fleet will actively orbit your mate's planet (limited to one-hour increments, up to 24 hours) before moving back home, as well as how quickly the fleet will fly there and back — if an attack is not imminent, then selecting a slower fleet speed saves hydrogen fuel.
In addition to the cost of traveling to the planet (which depends on the speed), initiating a Group Defend costs hydrogen for the time selected to spend in orbit: the amount is one-tenth of the fuel consumption rating of the fleet for every hour to spend. So, for example, defending a mate with 10 Hades for 1 hour (which is the minimum that can be selected) costs 250 hydrogen on top of the cost of traveling to the planet.
From your own Fleet screen you can also recall the Group Defending fleet at any time. Doing so during its orbital period will result in the fleet beginning its voyage home immediately. As always, Hydrogen fuel is effectively consumed at launch and is never refunded.
A maximum of 6 fleets may participate in a Group Defend. This means a group defender may have 6 players join the planet for a group defend. This is the defender plus 6 fleets, making 7 defending fleets.
Ships present at a planet during a Group Defend mission's orbital period will combat against any incoming attacks, INCLUDING those launched by the Group Defend fleet's owner and/or their allies and/or their buddies. No DSP is awarded to players who have fleets on both sides of a battle; the player’s share is forfeited.
This is the defensive counterpart to Group Attack.
In battle, each ship performs according to the techs (Armor/Weapon/Shield) of the respective owners.
Fleets traveling to a planet, in orbit around a planet, or returning after the selected Group Defend time has expired can be detected by an Oracle scan of the planet. However, fleets that are recalled before the time has expired are invisible to the Oracle.
Fleets in orbit also show up on a probe of the target. However, it is not possible to determine the battle techs of the Group Defending fleets from the espionage report; to know the exact strength of the combined fleet requires direct espionage of the owners of each fleet.
Group Defending a planet (or moon or Hephaestus) gives no knowledge of other defenders, incoming attacks, probes, or other activity apart from battle reports. This information must be obtained from the target’s owner or from an Oracle scan (where possible).
In SFC Nova and SFC Conquest, Group Defend is limited by Newbie Protection.
Starfleet Commander Conquest
In Starfleet Commander Conquest, a territory that is already occupied by a buddy or ally can be Group Defended in the same way as a friendly planet. Since occupying a territory is identical to Group Defending it (fleets participating in a successful attack on a territory are automatically converted to a 24 hour Group Defend for free), technically, up to seven fleets can GD a territory. (The effect is the same as one owner and six GDers, and is the same as the maximum number of fleets that can attack in a group.)
A territory that is already occupied by oneself can be Group Defended even if one has exceeded (or dropped below) the RSP limits for the territory.
Since occupying a territory is a merely a Group Defend (rather than some deeper form of ownership), GDing a territory does cause incoming attacks (attacks only) on the territory to appear on one’s Fleet page, and there is a special “Territory Under Attack” indicator that lights when any occupied territory is being attacked. The only indication of a territory being probed is the activity indicator (*) on the galaxy screen.
