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.
Everve
Sign up and log in using your Google account. In the top dropdown menu, select Advertise, then click Add Funds on the left. After topping up your account using cryptocurrency or supported payment processors, click Add New Campaign > Websites > Traffic exchange. Add your generated landing page URL with your referral ID at the end. The cost is $0.0012 per click, and you can expect over a 1,000 clicks a day at first, which will drop over time.
TimeBucks
Sign up with an email and password. Top up your account by at least $10 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.
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.
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.