In order to accept an invite, you will have to be in the main menu of the game itself. When you continue onto the game, go to the 'Co-op' section of the menu, and select 'Invite Friend' to your Co-op session. In order to invite a player, you will need to select 'Continue ' from the main menu of the game. This can also be earned by liberating a Fortress. If the host has liberated all 24 outposts, then the host can reset their outposts from the menu under Gameplay (Campaign), there is an option to 'Reset Outposts'. Also bear in mind that you cannot replay outposts that the host has liberated while in Co-op, so you will have to liberate an outpost that needs to be liberated. This can be earned with a random, but it's highly recommended to organize a boosting session, as you can take turns inviting each other while liberating an outpost. Not only is it in Co-op, but you will need to be invited into another player's lobby, as the host will always be Ajay. The only way that you can play as Hurk, is in Co-op.
This trophy is quite easy, all you have to do to meet the requirements to earn this trophy, is to liberate 1 outpost playing as Hurk. Note: You need to complete Act 1 in order to play the game in co-op.