It mentions that the US is to send a team of experts to help Russian investigation of the Boeing 737 crash. Would these be US government AAIB investigators, or experts from the Boeing Company?
Short answer: Both.
Unless the country in question forbids it, it is SOP for the NTSB (National Transportation Safety Board) to be involved in the investigation of any accident involving a US built airliner. Likewise, the manufacturer will send accident investigators that can also bring in Subject Mater Experts (SME) to assist as needed.
However the investigation will be headed by the local Russian accident investigation agency - the NTSB and Boeing reps are just there to assist.