Delilah, Ghostpoet and Lianne La Havas have also joined forces with Equals to fight against discrimination, violence and sexism.
Garai picked her Hours co-star Dominic West to be her equal for the project. The pair said: "We will know we are equals when there are as many female characters as male characters on our television and in films."
MTV's Laura Whitmore picked co-presenter Joel Dommett, who said: "We are equals because we both know and understand that Laura is way better at presenting than I am."
The cast of Hollyoaks have also starred in a short film for Equals. Bianca Hendrickse-Spendlove, who plays Texas in the Channel 4 soap, said: "We will know we are equals when every girl around the world has the opportunity to go to school."
Equals has also created a Facebook app allowing users to invite a friend to be their equal, which generates a special photo to share.
Helen Hector of Equals said: "Many young people think gender inequality is a problem solved, but even here in the UK 42% of young people know girls whose boyfriends have pressurised them into sex.
"In poor countries women are still fighting for basic human rights like education, choosing who they marry, and a say in decisions that affect their lives."
Annie Lennox, Katy B and Emeli Sandé are among the stars set to perform at the Equals Live Show at London's Royal Festival Hall on March 9.
Watch the Hollyoaks Equals film below: