Just go through the events in Emi's room and Agnes's events (including story events — over time, your relationship will improve and new actions will become available on the beach).
After you have completed story firstly after Agnes 1st scene, in Emi room "spent time" with her, to talk with her and trigger this scene after this you can unlock the option for the beach, for Agnes you have to complete more story/+Love path/ and some of her events after.
