Go to your friends list and tap on the [...] beside their name, then turning On the switches to change permissions. This should be done prior to their visit.
After changing, the friend will be able to claim from any and all processors you have and load ingredients in the accessible processors. The goods from the following processors will go to the backpack of the player who claimed them NOT the player who loaded them. Crops and livestock yield will go to the farm owner. XP points only goes to the player who claimed the items. Friends do not have access to the shed.
Depending on permissions, friends can rearrange and redecorate farmhouse furniture and walls.