Yes you can put coffee grounds around the hydrangeas plants.
Don't put the coffee grounds directly on the hydrangeas as it could possibly kill them but if you put the coffee grounds in the soil of the hydrangeas they will thrive much better.
The coffee grounds add acidity to the hydrangeas soil which helps them absorb the naturally occurring aluminum in the soil.
The coffee grounds also help keep the hydrangeas color vibrant and the hydrangeas will be much healthier as result.
I have some hydrangeas plants out in front of my house and use coffee grounds at least once per month in the soil of them and they thrive much better and have better colors to them.