What is the purpose of the invites? I'm assuming you're only handing them out freely because the project is still new and needs more users?
Purpose of invites is to improve your community growth score (apart from bringing more people into Merit

). That increases your chances at getting more growth rewards in MRT (the Merit Coin) and INV (the Invite Token)
Is there a determined amount of growth rewards per invite?
No, its highly dependent upon a variety of factors which all get bundled up into two different scores.
1) Community Growth Score where each invite does help boost your score, but only if the account that you referred in has a non-zero balance. Your score increases by 50% of the balance that is held by a direct referral. This score is compared across the entire community and is used to determine whether your account is eligible for a lottery style drawing and if your account is picked, you will get paid a growth reward. Typically the growth reward is under .6 MRT per time selected from the lottery. The frequency is actually what makes the difference in earning a little or a lot of growth rewards.
2) There is a staking side of the growth rewards as well, where the balance you hold is used to calculate a different score that basically acts like staking. Coins mature 50% after 7 days and are at 100% weight after 30 days. Your staking score is compared across all other community members as well.
Hope this sheds some light on the proof of growth rewards.