In compliance with an order from the Secretary of the Department of Interior, Yellowstone, Grand Teton and Glacier National Parks have expanded accessibility for electric bicycles.
E-bikes are now allowed everywhere traditional bicycles are allowed. They will also be allowed on pathways of the National Elk Refuge that connect to Grand Teton National Park. Just like regular bikes, they will not be allowed in designated wilderness, in areas managed as wilderness, or on over-snow roads during the winter.
By definition, E-bikes are low-speed electric bicycles with power assistance. Operators may only use the motor to assist pedal propulsion and the motors may not be used to propel a bike without the rider also pedaling, except in locations open to public motor vehicle traffic. All must obey speed limits and other traffic laws.
You can find additional information about the new policy here.