After airing on CBS for its first season, the rights to Supergirl were acquired by the CW, the same network that produces Arrow, The Flash and Legends Of Tomorrow, all of which have amassed audiences of respectable sizes. Since Supergirl is Kryptonian, it makes sense for her to be part of the upcoming Arrowverse mega crossover, which will see Earth threatened by an invading alien race known as the Dominators. However, according to executive producer Andrew Kreisberg, Supergirl’s world won’t merge with the rest of the Arrowverse, as she lives on a whole different Earth.
As per Kreisberg, any interdimensional travel will “not [be] for her entire world, but Kara will be traveling from her dimension to our dimension, ‘our’ being the world that The Flash, Arrow, and Legends lives in.”
It makes sense for Supergirl to mostly stay in her own world, as her powers are so above and beyond what any of the other Arrowverse heroes can do that a crossover would have a very weird feel to it.