Each friend can help you with 5 buildings (or less) in a day speeding up each building by 30 minutes.
Icon of friend will appear above one of this 5 buildings only - hover your cursor to this icon and all buildings speeded up by this friend will be highlighted. Then you can accept or decline these accelerations from this friend. Icon will move from one of speeded up buildings to another.
Please remember that one building can be speeded up by 5 friends only. You need to accept this accelerations after that.
Friends can also send you items you need to finish a task and help you earn Hearts and Stars.
You can also help your friends with 5 buildings a day and get coins, Energy and Excursion Tickets.
One can visit a neighboor's city again only on the next day (after 02:00 a.m. GMT).