You would be wise to go through a good cruise agent if you are going to book a group cruise. Make sure you have an idea of how many people will be going and if you feel others might join. The agent will work out a price with the cruise lines and if any perks will be included. Let the agent know what you are expecting from them. One important fact to remember is that not all group cruise prices are cheaper than booking individually. If your group is small it may be cheaper to book individually. What the group price in that case may have to offer is just some perks but you may pay extra for the perks. Get information from agent in writing.
A good TA will be able to discuss the plus and minuses of a group cruise with you depending on the size of the group.