The rallying banner will summon your Guards to you when used. It’s especially useful in raids, as Guards don’t automatically know where to find the raiders.
Note: Your Guards will not follow you outside the colony border. To bring Guards outside the border, set them to Follow you on the second page of their tower GUI.
Using the Rallying Banner
First, shift + right-click while holding the banner on a Guard or Barracks Tower. This assigns the Guards in that tower to the rallying banner. (Shift + right-click on the block again to remove the tower.)
Then, shift + right-click on any other block to summon the Guards! (Shift + right-click again to dismiss them.)
At any time, you can right-click while holding the banner to open a GUI, which will show the towers currently added and the number of hired Guards at each one. You can remove the towers from this GUI, and rally/dismiss the Guards as well.