Heroic monsters squads are very strong, which means that it could be problematic to defeat them on your own. To fight such powerful enemies the game has an option of group marches.
A group march by definition means that several armies of various players participate in an attack. To create a group march it is enough to know who will head the attack and then simply send reinforcements to this player’s city. The player heading the attack will be able to control all of the troops and send them to a march. Rewards will be distributed according to each player’s contribution to killing monsters and total army strength.
Note that unlike regular marches, you cannot speed up a group march for more than four times. Special march speedups are needed to accelerate a group march. Their cost depends on the march size (how many players participate in a march) and is 5K gold for a maximum of 5 members and 20K gold for 25 members.