if (input1 == "список")
{
var chatId = message.Chat.Id;
var membersCount = await client.GetChatMemberCountAsync(chatId);
var random = new Random();
var randomIndex = random.Next(membersCount);
var members = await client.GetChatMemberAsync(chatId); //ошибка здесь
var randomMember = members[randomIndex];
await client.SendTextMessageAsync(
chatId,
$"Random member: {randomMember.FirstName} {randomMember.LastName} ({randomMember.Username})");
}