The initial 12-month appointment will see the agency, led by director Stuart Fox, and Crossrail's in-house team work together on national, regional and trade media relations, stakeholder relations and drumming up support for the project among the business community.
The company has been allocated £154m from the Government this year to carry out feasibility studies on the project and take preparatory steps towards building the tracks.
It is proposed that two routes will operate: an east-to-west line, Crossrail 1, and a north-east to south-west line, Crossrail 2.
Details of proposed routes for the first line were released earlier this month.
Following campaigning from London mayor Ken Livingstone, this line will also include a link to Heathrow Airport.
This will help the transport network cope with additional passengers that will use the airport following the decision to give the go-ahead for a fifth terminal.
Crossrail communications manager Rupert Brennan-Brown - to whom the agency reports - said the importance of the communications function was heightened during the feasibility period: 'We already have considerable support for the scheme.
'We now need to build on it and ensure the scheme remains front of mind during the feasibility and development period,' he added.