Gollum falling into Orodruin and thus destroying the Ring was planned by the Fellowship from the start. Or more specifically, Gandalf, Legolas, Aragorn and Frodo.
When Gollum was in captivity in Mirkwood, Legolas helped him to escape. He was of course above suspicion, being an elven princeling, but his guilty
conscience bothered him and he offered to go to Rivendell to tell Elrond
that Gollum had ‘escaped’.
Gandalf was responsible for the whole idea, and dropped the rest of the Fellowship imperceptible hints such as “My heart tells me yet that Gollum has some part to play…for good or ill.”
This was also a warning to Frodo: ‘Don’t you mess up, or things will go ill, especially for you.’
Frodo played his part by defending Gollum from Sam and thus allaying his suspicions, and also luring him to Mount Doom. When he tried to keep the Ring for himself and Gollum bit it off him, Frodo had a desperate attempt to salvage the situation and pushed the hapless Smeagol of the precipice when Sam wasn’t looking.
Sam was not included in the plot, because Frodo feared it would lessen his opinion of him, and also Gollum might suspect if they were both so eager for Gollum to follow them.
Merry and Pippin had an idea, and their conspiracy earlier in the book was a subtle and ironic hint towards darker matters.
Gollum, naturally was unaware of the conspiracy, and was the witless and somewhat bewildered victim of the whole scheme.