To go with or attend as a companion or associate; to keep company with; to go along with.
To perform an accompanying part next to (another instrument or musician).
To perform an accompanying part or parts in a composition.
To supplement with; add to.
To be found at the same time.
To keep safe; to defend; to guard; to prevent harm coming to.
To book a passenger on a later flight if there is a chance they will not be able to board their earlier reserved flight.