1. Have a strong referral system in place
You should be able to incentivize users to advocate for the app and drive more installs. Larger incentive can be given for more effective referrals (e.g. 10 Euro for sign up, 5 Euro per driver oriented referral). You could also promote itself through its existing partnership with GetYourGuide.
2. Build integrations with platforms used by users
Examples of this are building Lyft, Uber, Sidecar, and maybe even Getaround integrations. This is an incredibly efficient way to scale a business by tapping into the high concentration of users being served by these existing platforms. The advantage of using multiple rideshare apps is that it creates a viral loop for the product - and you will likely be exposed to a much larger audience through these partnerships.
3. Partner with different organizations
You can partner with CSAs, co-working spaces, third-party car companies etc. to understand their car fleet composition and usage at different times of the day. This information could be used as a base to leverage your popularity to incentivize Carpooling.
4. Incentivize users to refer their friends to
By rewarding the first referrer with a free ride or discount on your platform, you are able to create a one-sided viral loop.
5. Have an affiliate program
It can be for drivers to share your platform(website/app etc) with their friends. So if a driver signs up new client and gets $500, he/she can share some of it with their friends via Facebook or Snapchat.
6. Provide a referral program
That rewards the person who invites their friends or people they know to use your platform.
7. Set a high minimum of friends (500+) to unlock all your services
This will encourage more signups by people who need help coming up with 500 friends.
8. Integrate with a social platform
Like Facebook or Snapchat etc. It is indisputable information for almost all businesses find out where users may be going that they would like to have a quick car to pickup at the location. This would allow them to jump in the car immediately and grab it from the designated area.
9. Offer different payment methods
Like "Pay For My Ride" and allow users to make payments or transfer payment for a ride they pre-booked.