FAQ > GAMEPLAY > Can you "group" units?

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Yes, the buttons in the bottom left are used for grouping. Hold the group number button you want and then select/deselect units to add or remove them from the group. Tapping that group button in the future will then select these units. Double-tapping the group button will clear the group.

 

The "multi-select" button, which is to the left of the group one button is described here.