Sharing units between groups
Move units between your groups, split a group in two, or merge two into one.
Right click one of your groups while another is selected, and their two panels open side by side once they are close. Pick units in one panel and right click the empty slot in the other to move them across. It is free and instant.
You can also split some units off into a new group, or merge two groups into one.
Units cannot walk across a river to change groups, and cannot leave a group that is locked in battle, though fresh troops may join it.