An ACS group has a limited number of mission positions. Under the current mechanic, the default capacity is 4 mission slots, so only a limited number of fleets can participate in one joint attack.
How does this rule work?
Each valid joined fleet occupies a group position. Once every group slot is used, another fleet cannot be added merely because it is flying to the same target.
What exactly does the server validate?
ACS group slots are separate from a player’s normal fleet-slot limit. A player must be able to dispatch the mission and the group must also have capacity for the additional participant.
How does this affect the mission?
For a large operation, decide in advance which fleets really need to join. Sending small or accidental fleets can consume scarce group positions that would be more useful for stronger participants.
When does it matter?
ACS does not create a separate, looser combat system. A joint attack still uses normal player protection, attack limits, target validation, fuel and fleet-slot rules. The group only synchronizes several valid fleets into one battle context.
Common mistake
Before joining, verify the leader, target, object type and planned battle time. Most problems come from trying to join after the target changed, from outside the required alliance, or with a fleet that cannot arrive at the required time.
Related FAQ: Who can join an ACS attack? · Can you change the target of an ACS group?