An offensive ACS can be joined by the group leader and players who belong to the same alliance as that leader. A buddy relationship by itself is not enough to join the joint attack.
How does this rule work?
When a player tries to join, Inter-War validates the relationship to the leader, the ACS group type and the current mission state. The fleet has to join the actual group rather than merely fly to the same coordinates.
What exactly does the server validate?
If the player is not in the required alliance, the server should reject the join attempt. This differs from defensive support, where an accepted buddy can be eligible to help.
How does this affect the mission?
In practice, agree on group composition before the first fleets are launched. Alliance membership changes or assuming that buddy status grants the same rights as alliance membership can prevent a join.
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: How many slots does an ACS group have? · How does ACS arrival-time synchronization work?