I asked because the 83(b) election is traditionally required in a restricted stock purchase agreement. If the agreement in place when the shares were purchased was not restrictive, that is, all the elements of the purchase were complete, then the 83(b) deferral technique may not be valid. The Section 83(b) election can be made when the property received was nontransferable and subject to a substantial risk of forfeiture. You did not mention any restrictions on the original purchase, so I believe you are out of luck.
IRS would consider the purchase complete when funds transferred, since nothing else was required to vest. The purchase would be a taxable transaction at that time. There is not a remedy for the late election you seek. I asked about the FMV of the shares, and the purchase price assigned, to see if there was a discount at purchase.
Adding a vesting schedule after there was a payment made and there was no substantial risk of forfeiture is not a viable excuse for the 83(b) relief, since the purchases were completed without vesting.
I can suggest a repair technique, which could be an exchange of those shares for new shares subject to the vesting schedule. The founders may be subject to tax on the original purchase of their shares for $.00001 per share, compared to the fair market value of those shares at purchase.
The 83 (b) election could then be made by the founders within 30 days of the exchange of their shares for the new restricted ones. Copies of that election are then included in the personal returns of the purchasers.
Thanks for asking at Just Answer/PDtax.