This guide will let you know how to CO-OP In Lost Ark.
You will need to go to the social menu, click on the friends’ tab from the top, and find a search bar. Now you have to search for your friends with their usernames. After that, click on the friend request button, and then you have to enter your friend’s name and click the OK button.
When you accept the friend request, you need to go to your friends’ list, and then you have to right-click on their name and select the party invite option to send them an invitation. You also need to make sure that you are not in the prologue.