My thoughts on this :-
Limbo is the realm between Life and Death. It can be accessed with the Rinnegan's Outer Path Ability which can control the boundaries between Life and Death. There are two ways to enter the Limbo Realm which might be a part of the Outer Path :-
1) Have a near death experience.
2) Have the Rinnegan.
It's possible that Naruto and Sauske met the Rikudou in the Limbo Realm. Moreover it seems that with Natural Energy, you can partially enter the Limbo Realm and sense Limbo Entities.
With Limbo Hengoku, Madara sends a part of him to the Limbo Realm by the Outer path's ability and uses it to influence events in the real world.
EDIT: Sauske can enter the Limbo Realm due to his Rinnegan.