Hey Aljo,
To answer your questions for future reference/help with others,
- In this case, we were all PSN
- We tried multiple ways, i.e. via menus or via an invite
- I did power cycle my equipment before posting here
However, I believe I may have resolved it in the short term. It appears there's another connectivity issue specific to AT&T users. Ultimately, I tried a few different things I was seeing in posts about other games with the same kind of symptoms (can only not connect in some instances, no error code displayed, etc.) and what worked for me was setting my primary DNS on my console to 8.8.8.8 and secondary to 8.8.4.4 -- it appears there's some hangup between the servers and DNS settings. All is well for now. Anecdotally, this change also generally sped up connections and navigation between different online modes in-game across the board.
All good for now--thanks Aljo!