A320 Captain reported that a gate agent opened an armed aircraft door without notification to allow additional passengers to board. Escape slide did not deploy.

Narrative

Aircraft X to ZZZ [Airport] following a gate return for maintenance and an aircraft swap. When the First Officer and I arrived at the new gate; boarding had already began. For this second time doing the same flight things were flowing like normal and not too rushed. The gate agent asked if she could close the door; I asked all crew if they were ready and confirmed with the gate agent there were no passengers standing by. We closed the door and shortly after the First Officer and I began the pre flight checklist with all doors armed and I also heard the Flight Attendants begin their safety briefing. Then without warning we simultaneously received a call from the Flight Attendant and an annunciation that the main boarding door was open. Our purser said that she waived at the gate agent through the port hole when she saw her but she simply went straight to the door and opened it with no notification or knock. The door was armed. When we asked why she opened the door she said she wanted to get a few more passengers on board....no crew had been notified of this. Fortunately the Airbus slide won't deploy in this situation but I believe this is still a safety concern and should not be done in this way. The gate manager came down and told me the gate agent was new and didn't know she couldn't just open the door....to which I wondered why she was allowed down to the gate alone in the first place....not to mention she was probably overwhelmed as I noticed she was also working our flight alone. I did debrief [with] both the manager and the gate agent about my safety concerns for them and the aircraft; and that they should follow normal arrival procedures at a minimum; but really needed to also make certain the crew is aware of and ok with the boarding door being opened before just simply doing it. For the employee involved I'm in support of some grace here; but strongly recommend extra training.