Whenever my friends try to invite me to a party I don't get the Accept/Decline popup.
I do get the popup in the bottom right corner of the screen from steam that says, "friendName has invited you to play Dota 2".

Sometimes I can open the steam overlay and accept the invite by clicking the lighter tab above the chatbox that says, "friendName has invited you to play Dota 2" and then "CLICK HERE TO JOIN".
This does not always work and I often have to resort to inviting everyone to my party.

I looked for solutions on google and searching through the forums but I haven't been able to find anyone with the same problem. The problem started around the same time as the Wraith-Night update.

Thanks ahead of time!