Parking in the city centre: Most possibly you will have to pay for the parking. However, the maximum fee is € 12 for 24 hours. This is a viable option if you would like to walk around the central area and you/your friends will not use the public transport.
Parking in HVV P+R (Park & Ride): HVV offers free parking lots outside of the city centre. The idea is that you leave your car there and use the public transport to get around. If you and/or your company merely would like to travel around the city centre on foot the first option is cheaper and makes more sense.