As the name of the game might suggest, Pokemon: Let's Go, Eevee does have connectivity with Pokemon GO. With it, trainers can send their Pokemon from Pokemon GO to Pokemon: Let's Go, Eevee, allowing trainers to capture said Pokemon within the latter game in an area to be called GO Park. In GO Park, players playing Pokemon: Let's Go, Eevee can interact and capture the Pokemon they sent from Pokemon GO.
Unfortunately, only the Pokemon from Bulbasaur to Mewtwo (all Kanto Region Pokemon outside of Mew) can be sent from Pokemon GO to Pokemon: Let's Go, Eevee, including the various Alola Region Forms for the 1st Generation Pokemon that have one. Furthermore, Pokemon sent from Pokemon GO to Pokemon: Let's Go, Eevee cannot be returned once sent to GO Park. (But you will receive Candy in Pokemon GO for sending your Pokemon to Pokemon: Let's Go, Eevee.)