As of January 2018, there is no place for adding contact info for background actors. 😕 You can list extras breakdowns on the call sheet, of course, but there is not a built in way for adding specific contact info for extras. We're looking at adding this feature in the future, since it would obviously be nice to have.
For now, I'd recommend working around this in one of the following ways:
Add extras under the cast member section and just add "Extra" to the name the character, such as "Extra - Pedestrian". This is nice because then you can select them and publish the call sheet to them like you would with cast members. You could also tab them in specific scenes using their character number which could be something like "101", "102", "103", etc.
Another option is to send the call sheet to extras separately than then rest of the cast and crew by just downloading the call sheet PDF and emailing it to all the individuals who are playing extras.
We apologize that there's not a better way to handle this at the time, but we are working at bringing that into the next version!