In the task group orders window, first select the task group containing the troop transports. Then click the population center that holds the troops in the left field. Then click in the order field, the center field, on "load ground troops". If you want to load an entire brigade, select the brigade headquarters in the window that pops up in the bottom right, and make sure "load whole unit if HQ is selected" is ticked, then click add order.
How do I transfer GUs to a ship? I have the troop capacity on the ship but as soon as I select the load option on fleet orders it deselects the brigade and I get the "select a ground unit" prompt.