Basically your frend built "just what you asked for but not what you wanted". You have discovered the limitations of a single speed/fixed gear bike and that is they only have ONE gear.
I agree with the other posters and consider you have three choices:
1. Spend more than it's worth and convert it back to a derailleur bike or fit an internal geared hub.
2. Buy a properly geared new or used bike for your group rides and sell your current bike or keep it as a beater or casual ride.
3. Keep what you have and accept its limitations.