You can easily play multiplayer co op with your friends. But you can only access co op features once you’ve left Dutch's Island. It must also be noted that console players will need PS Plus or Xbox Live Gold membership to play Far Cry 5 online in co op play mode.
You may start enjoying it by opening the in-game menu and selecting the Online option. You can then click on 'Invite Friends' and choose the friend you would like to invite. Friendly fire will be turned off by default; but you can turn it on for an extra challenge.