Copy Pasting from Super Community Member disifi's post on Discord:
"- Invites are Airdropped in 3 different ways
      - 50% awarded based on your CGS (Community Growth Score)
      - 40% awarded to new addresses (Max 1 per address)
      - 10% awarded to random addresses
-  1 out of 10 blocks an invite is awarded to a miner
- Minimum of 144 invites airdropped per day. Grows as needed.
- Number Airdropped based on overall growth, more growth means more invites generated"

In fact, PoW miners don't earn a lot of INVs (invite tokens) from their mining work per se.

There are a number of ways to earn/get INVs:
1) Payout from pools. All mining pools give out INVs to their miners. Most pools distribute INV based on hash power. Parachute Pool is more favorable for small-time and hobby miners (make sure to join their TG and fill pinned form if you mine with them). Details on mining pools: https://merit.disifi.com/pools
2) Asking your inviter for some. It is in the best interest of your inviter that you grow your network since it improves his CGS.
3) "Mine" an invite. This is random and the probability of it happening is entirely based on your CGS.
4) The last method is to buy OTC on Discord (with someone from the Super Community acting as escrow). Though this is the last resort if you can't get any INVs from the above methods.

Edit: I see Kamo has already answered this while I was typing. His point #2 is another way to get some INVs.
Edit #2: Thanks for the merits sockpuppet1911