Due to excessive crowds the saffs (Salaat rows) extend outside Musjid Nabawi as well as Musjid Haraam.

Answer

As long as there is continuity of the saffs from within the Musjid, you may follow the Imam. However if there is a break in the saffs once it overflows outside then the Salaat behind the Imaam will not be valid for those who are performing outside the Musjid. A gap of 2 saffs is regarded as a break in continuity and if this is found outside the Musjid then the Salaat of those behind the break is not valid behind the Imaam.