Can't Invite or Join Friend in Diablo IV

In Diablo IV, you can invite or join a player on your Battle.net Friends List by following the steps below:

1. While in-game, open the Social menu by pressing O
2. In your Friends List, right-click on the player you want to invite or join
3. Select Invite to Party or Request to Join Party

Both players must be in-game and on the same character type (Eternal/Seasonal) to send an invite or request to the other. In addition, if your friend has Appear Offline selected as their status on Battle.net, you may not have the option to invite or join them.