Return a list of all Active Directory groups a user belongs to in string[ ]

This should do the trick.

using System.DirectoryServices.AccountManagement;

public static string[] GetGroups(string username)
{
    string[] output = null;

    using (var ctx = new PrincipalContext(ContextType.Domain))
    using (var user = UserPrincipal.FindByIdentity(ctx, username))
    {
        if (user != null)
        {
            output = user.GetGroups() //this returns a collection of principal objects
                .Select(x => x.SamAccountName) // select the name.  you may change this to choose the display name or whatever you want
                .ToArray(); // convert to string array
        }
    }

    return output;
}