Don't worry about how many friends join you, as the game will fill the remaining character slots with bots. (bitcultures.com)
You can pay real money for extra character slots, as well as boosters to enhance your experience and resource gains. (gap4.com)
One issue I had with the game that I didn't got into as much detail as I should have is the fact you have 6 classes and 6 character slots for your party. (tech-gaming.com)