A developed economy which has saturated the capacity of its consumers to obtain credit and repay debt can grow in other ways than just increasing the number of people participating. 

It could continually increase its export of goods and services without increasing its imports; that is, it could continually increase its balance of trade.  It could also generate increasing economic activity within other economies, and repatriate the profits.

In both of these examples this activity amounts to increasing the number of people participating in the economy, because it is commandeers the economic activity of people in other economies.  Of cpurse, In both examples the activity must also be continually increased to generate continuous economic growth. 

Both these methods are commonly used to grow an economy, even though they amount to taking economic growth from another economy; but increasing population to increase participation in an economy is usually much easier to achieve.

Neither of these methods are relevant when we are considering the effects of growing population and economic growth on the whole Earth, as neither of them are relevant to the global economy, which is a closed economy: there are no other economies for the global economy to trade with or to infiltrate.

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Population growth and economic growth