PlayStation - Find A Missed Invite & Accept It
Don't worry if you miss the invite notification, you can still join your Rival match. You can find the Invite in the Notifications tab. To locate the Notifications tab, press the PlayStation logo button to bring up the main PlayStation menu. Use the left stick to navigate up and over to the text bubble icon with an “i” inside the bubble and press “X”.
Once in the “Notifications” menu, you will see your latest missed invite from your Rival opponent. Move down to the missed “Invite” and press “X”. This brings up the “Invitation” screen. Use the left stick to move down to the “Join” box at the bottom left of your screen and press “X”. You will be automatically taken to your game.