Once you have reached the end of season 1, 2, or part way (8-11 races when your partner's negotiation comes up), you can go to the drivers market to hire someone. It has to be someone with the acclaim 15 or higher (example: Lewis, Bottas, Max). When you view the stats of the player you want, press
again to approach. From there it'll be straight forward to go along, if they accept that is.
Note that if you aim to unlock this at the end of a season, it must be before the 'end of the season' in red on the calendar.