pitch in
Pitch in (verb) means to join in and help with a task, especially alongside others and without being asked. Example: “If everyone pitches in we can clear the hall in twenty minutes.”
How to use pitch in
Learner’s notesIn plain EnglishTo join in and help, or to put money in with everyone else.
Informal, separable only by an adverb: 'pitch right in'. Note that 'pitch into' someone means to attack or criticise them, which is quite different.