How to convert functions of a class that is not instantiated into delegates?

I have provided a minimum code below to convey my situation. I just want to figure out a way to create a delegate function for add(IClient client) in ClientList.cs and consume it in Program.cs

I have provided comments in the code below to better explain my situation. If you need any other information let me know, I'll update it here.

Program.cs

namespace sample
{
    class Program
    {
        static void Main(string[] args)
        {
            Program p = new Program();
            p.Go();
            Console.ReadKey();
        }
        
        private void Go()
        {
        
        //Code logic
            
            using (ClientList clientList = new ClientList())  //Instantiated an object of the ClientList class
            {
                //Code Logic
                
                //Make use of functions from the object instances here.
                clientList.add(client); //Works
                
                //ERROR : BUT, the below does not work . What am i doing wrong here.? Why is it not accessible here ?
                clientList.add
                
                
                
                clientList.Sort();
            }
        
        
            
        //Code logic
        
        
        }
    }

}

ClientList.cs

namespace sample
{
    
    public delegate void addItems(IClient client);   //ERROR in editor. (Red underline on addItems.)

    class ClientList : IDisposable
    {
        //Member variables
        private List<IClient> clientlist = null;
        private string time;

        //Member Properties
        public string Time { get => time; set => time = value; }

        //Default Constructor
        public ClientList()
        {
            clientlist = new List<IClient>();
        }

        //Member Methods
        public void add(IClient client)
        {
            clientlist.Add(client);
        }

        //Indexer
        public IClient this[int position]
        {
            get { return clientlist[position]; }
        }

    }
}

UPDATE #1:

The red underline error is as follows: CS0059: Inconsistent accessibility: parameter type 'IClient' is less accessible than delegate 'addItems'