There are two types of currency in the game. Chips are the main game currency, and pents are the elite money, the desired prize of everybody. Last are needed for the elite improvements, special modifications of spares and number of elite services, chips are spent on everything else.
Players who attract friends to share the game receive special currency, credits. You can get this currency not only for the invitation itself, but also for the significant achievements of your friends. For example, you’ll get much more for 10th level of your friend than for the invitation to 50(!) people.