using System;

using System.Collections.Generic;

using System.Linq;

public class Program {

    private static Dictionary<string, string> specialPlurals = new Dictionary<string, string> {

        {"Witness", "Witnesses"}

    };

    public static string Pluralize(string singular) {

        if (string.IsNullOrWhiteSpace(singular)) { return singular;}

        if (specialPlurals.TryGetValue(singular, out var plural)) { return plural;}

        return $"{singular}s";

    }

​

    public static void Main() {

        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 grouping = parties.GroupBy(

            party => party.Type,

            (type, group) => $"{Pluralize(type)}: {String.Join(", ", group.Select(party => party.Name))}"

        );

        var result = string.Join("; ", grouping);

        Console.WriteLine(result);

    }

}

​

public class Party {

    public string Type { get; set; }

    public string Name { get; set; }

}