I'm assuming two things here:
First, that you did not cause this accident and was not there. Second, that you have insurance.
Assuming the above two things are correct, you should take whatever notice you received and forward it to YOUR insurance company. They will deal with this, and will deny the claim assuming my first assumption was correct.
Here's "why" you do as I instruct:
If they decide to sue you, and you DO have insurance, your policy contains a clause (condition precedent to coverage) that you TIMELY report any accident, loss, etc. This will likely include the notice you have received. If you do not report it timely, your insurance carrier can DENY coverage when and if you do ask to be defended.
If you do NOT have coverage, you can ignore the notice and take your chances. The letter is most likely from the subrogation department and they play a numbers game. They are also much more interested in people with an insurance policy than someone with no insurance. As to the latter, you could, if you don't have insurance, simply reply that you do not, and tell them to fuck off. They can sue you, but they will need to retain counsel, and that itself will cost money and for a person without insurance, it's the law of diminishing returns there.
Fill in the blanks, and I can instruct you more specifically.