Currently the beta only includes multiplayer.
I'm not sure what the problem is joining a game (other than the obvious), but as BlueStar427 said you can start a game (Create lobby), then add 1 to 3 AI, change the factions if you wish (which determines if they are enemy or allies to you and each other), and then either start the Multplayer Campaign or just play a Skirmish (one RTS battle), selectable above the map.
If you wish to get a feel for the game mechanics, you can set the enemy AI to 'None' difficulty, so they will not build or recruit, allowing you all the time you wish to do whatever you want before attacking (they will defend themselves at that point).
Welcome to the forum.
