Discord.Net/docs/guides/commands/samples/intro/groups.cs

[Group("admin")]
public class AdminModule : ModuleBase<SocketCommandContext>
{
    [Group("clean")]
    public class CleanModule : ModuleBase<SocketCommandContext>
    {
        // ~admin clean
        [Command]
        public async Task DefaultCleanAsync()
        {
            // ...
        }

        // ~admin clean messages 15
        [Command("messages")]
        public async Task CleanAsync(int count)
        {
            // ...
        }
    }
    // ~admin ban foxbot#0282
    [Command("ban")]
    public Task BanAsync(IGuildUser user) => 
        Context.Guild.AddBanAsync(user);
}