In order to host without Alloy, you will need to do port forwarding on port 6112, exactly the same process as you would do if you were to set yourself up to host in Warcraft III.  With alloy, you shouldn't need to do any of that.

Now, how many players are online and how many of those choose to join your game is another matter.

There is a special IP address you can type into your URL bar (in whatever browser you're using) to access your router:

192.168.0.1

From there, your router takes over and will display its menu via your browser.  Most routers have password protection, but usually the factory default username of "admin" with no password will get you in (if not, consult your router's manual).  Once you're in there, you'll have to navigate through your router's menu to find the port forwarding option.  You'll need to create an entry that matches your computer's network IP address to the port number 6112.  Usually this is just a matter of filling in the right values, and the router may even provide you a drop-down list of all computers connected to your network for you to choose from.

As an aside, only one computer on your network can be set up for port fowarding of a specific port at any given time, so you have to pick which computer has hosting power.

However, if you're running alloy you don't need to do any of this.

 Is it that people are choosing not to join your games or that they cannot join your games?

If people can join your game, but aren't, I'm guessing there's an issue with the map or the kind of game you want to play. Locked team games are most popular, and very few people want to play with pirates on. People usually prefer the fastest game settings available, though with diplomacy, there may be a lot of "fast" speed holdouts. With larger games, people usually prefer single-star to multi-star maps. Also, most of the MP community isn't interested in fighting the AI - they want all PvP. Usually, people running comp stomps organize them with friends, rather than just leaving them open and waiting for people to join. I know for a fact that I don't join games with AIs listed, even if they're open, because if I want to fight AI, I'll do it single player. 

So, unless you can organize with friends ahead of time, the best way to get players is to host a locked teams 3v3, 4v4, or 5v5 with pirates off and the fastest speed settings. Even easier, though, is to just join other games that are up already.

Hope this helps,

Yakov