You can drag tasks into a task group, to make them part of that group. See Groups and Subtasks.
To drag and drop a task, drag the Task Icon
with your mouse.To drag and drop several tasks at once, select them first, then press Ctrl and drag and drop one of the selected tasks.
You can drag tasks onto the Filter Panel to assign properties to them. You can use this feature to assign the same property to several tasks at once.
You can drag and drop goals within the Goal Tree. See Using goals.