While in game, simply press Shift+Tab. Then click on view players. After this you want to click on recent games because the current game list is bugged and doesn't show anyone.

To invite someone, from that view, click on their profile link, then on the left click on invite to my group... and select warcraft-source.net from the list.

Try and invite as many people to the group per day so more people have an awareness of the server and how great it is... its also a great way of everyone being informed of changes to the server or events.