If an employer is offering life insurance where the employee doesn't contribute anything, would that be okay to opt-in?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
Life insurance is not permissible in Islam. It contains speculation (gara) and falls under the prohibition of consuming haram wealth. ِAllah says. “Do not consume your wealth amongst each other through false means.” 
Taking this into consideration, it is not permissible to benefit from the contribution of the employer that is invested into life insurance. The wealth retained from haram means itself is haram.
Only Allah knows best
Written by Maulana Nabil Khan
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Surah Nisa, Verse 188