The cash amount that you see in the "Investable cash" section of your Home screen can be used to manually purchase more stock in the companies you already own on Grifin:
Investable cash is the sum total of cash from the following places:
1) Quarterly dividend earnings that are paid out to you by all the stocks you own on Grifin that offer a dividend (yes, even if its just $0.01 per quarter, you'll still get a dividend!),
2) Referral cash rewards for referring your friend(s) to Grifin (or being referred), and
3) Any manually deposited cash you've transferred from your bank to Grifin via the "Add money" button on the Home tab!