Where to Promote
You may promote your GrassPromo landing pages in any legal, non-spammy manner. If you need some ideas, I was able to reach the highest Titan tier in Grass by advertising cheaply on the sites below.
TaskPulse
Sign in using your Google account. In the top right corner, click your account name, and in the drop-down menu, select Advertiser. On the left menu, click Add New Campaign > Surfing & Video Views. The cost is $0.0013 per 15-second view, and the number of views will depend on whether you want visits from all countries or only select few. Funds can be added via credit cards, Google Pay or Apple pay, and crypto.
Coinpayu
Sign up with an email and password or Google account. Top up your purchase balance via cryptocurrency or Payeer. Click Advertise > Create New Ad and fill out the details. The lowest amount to start with is 1,000 clicks at 1.4 coins per click (approximately $1.4). The cost per click will increase if you use geotargeting.
adBTC
Sign up and login with your Google account. Click Advertise > Surf ads and add your landing page. Deposit some funds into your advertising balance through Bitcoin or Payeer. Price per click starts at 3 Satoshi (0.00000003 BTC).
HeedYou
Sign up with an email and password. Create a campaign by clicking Advertise > Create New and choosing an exposure time (Glance is cheapest at $0.001 per 5-second view). Pay for the desired number of views and your campaign will begin immediately. Note: in addition to campaign views, you'll also receive bonus credits, so don't forget to add an URL to assign them to by clicking the "You have X bonus credits" button.
TimeBucks
Sign up with an email and password. Top up your account by at least $20 by clicking Advertise > Funds. Now click Clicks > Create Campaign and fill out the fields (URL being your landing page URL). It is recommended that you use the lowest cost per click ($0.002) because you will receive thousands of clicks per day.
Traffic Exchanges
Traffic exchanges involve clicking for credits and then using those credits to advertise your website. These might get you some referrals, but not as quickly or easily as the previously mentioned sites.
You can also buy credits instead of clicking tediously, but the results often aren't great. The biggest saving grace of traffic exchanges is that they can be used for free.