At his sentencing, David’s actions were described as “a fraud of the utmost gravity with a devastating effect on others.”
The court heard he used the money to fund his gambling habit, pay for family holidays and pay his children's school fees.
David's friends were among the victims.
He stood down as managing director at wealth management company HBFS Financial Services last year amid a police investigation into the fraud.
The victims invested between £20,000 and £750,000 in the scheme. The fraud ran "in parallel with legitimate HBFS business", police said.