Guagua from Las Terrenas to Bayahibe???

Not directly

Take any of the buses to Santo Domingo, get off at their terminal on Barahona street. Ask the terminal employees where the La Romana bus station is (not far), go there and catch the next one to La Romana. When you arrive to La Romana transfer to the buses for Bayahibe. At every step you may ask the "cobrador" for directions and instructions.

If you're feeling adventurous, take the bus Las Terrenas-Santo Domingo and get off at the crossroads to the airport (at the end of the new highway). Catch a motoconcho to the other side of the highway, where traffic goes eastward towards San Pedro or La Romana. Flag down one of the buses to San Pedro or La Romana. Once you get to La Romana go to the stop by the market where the buses to Bayahibe are.

You can take guagua to Nagua intersection, from there another one going south towards SD airport, then switch to one going to San Pedro, then La Romana and then Bayahibe. It might be fun but a bit longer travel then with the buses. Guagua to Bayahibe in La Romana is close to Jumbo supermarket.