using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Dictionary<string,string> plurals = new Dictionary<string,string>{

            ["Plaintiff"] = "Plaintiffs",

            ["Defendant"] = "Defendants",

            ["Witness"] = "Witnesses"

        };

        List<Party> parties = new List<Party>();

        parties.Add(new Party {Type = "Plaintiff", Name = "Eilean Dover"});

        parties.Add(new Party {Type = "Plaintiff", Name = "Bea O'Problem"});

        parties.Add(new Party {Type = "Defendant", Name = "Anna Graham"});

        parties.Add(new Party {Type = "Witness", Name = "John Doe"});

        parties.Add(new Party {Type = "Witness", Name = "Rosa Shore"});

        var result = string.Join(", ", parties.GroupBy(x => x.Type).Select(item => {

            var prefix = item.Count()>1 ? plurals[item.Key] : item.Key;

            var people = String.Join(", ", item.Select(x => x.Name));

            return $"{prefix} {people}";

        }));

        Console.WriteLine(result);

    }

}

​

public class Party

{

    public string Type { get; set; }

    public string Name { get; set; }

}