Issues with Multiplayer / Co-Op / Parties
Unresponsive Invite Friend button
"Invite Friend" button in the Lobby Settings, on the right board while in Multiplayer - it relies heavily on the Meta ecosystem. Please make sure that both - the Quest's system firmware, as well as the Oculus Social component (feature under the 'people' icon, that can be accessed via the quick menu as seen below) are up to date, before trying again.
Issues while joining a friend's session or private lobby
Please make sure that by any chance you are both not logged in to the same cloud account, as this could theoretically lead to issues.
Also - consider utilizing Meta's Party feature, which may make things even easier:
How to play with friends
Generally, from the host perspective, whole process looks like this: Private Lobby / Create lobby / Create a session / Co-Op / (select mission) / Create / turn on the "Ready" checkbox on the left board, bottom left / Start game
Looking from the other player's side - only bolded steps are different, as it is necessary to join an already existing lobby. What's more - the Host can start the game when all participants have declared their readiness.