On-demand mobility services and Transportation Network Companies (TNC) are transforming urban mobility by providing more flexibility and improved level of service to users. However, they also raise concerns about their impact on congestion, vehicle miles traveled (VMT), and competition with transit. Considering the popularity of TNC services, increasing ride-pooling is, potentially, an important means to address these concerns. While companies attempt to promote ride-pooling with pricing strategies, evidence suggests that shared trips are a small fraction of all trips. We review the literature on the impact of TNC on congestion and ridership of alternative sustainable modes and discuss opportunities to improve sharing performance. We examine two such opportunities: Alternative operating models that use advanced booking to improve shareability; and collaboration with traditional transit. In the first case, we use a large TNC data set to assess the benefits of advanced booking in terms of increasing shared trips and reducing VMT. The results suggest that even with short advanced request horizons, significant benefits can be realized, with little deterioration of the level of service that customers experience. In the second case, we review results from various partnerships between TNCs and transit agencies and highlight the main characteristics.