DHC 8-300 flight crew reported witnessing unsupervised passengers walking out to the aircraft for boarding. The agent had not alerted the crew of the boarding process.

Narrative

Aircraft had a maintenance open write up for the main cabin air stair door and maintenance was just completing their work. Also; at the same time; the weather at the destination was below dispatch minimums and the dispatcher did not publish the release until it improved. I noticed a stream of passengers walking out to our aircraft unsupervised. I walked out to direct them back into the terminal and noticed the gate agent to stop boarding. Never was our crew asked whether we were ready to board; nor was there a flight release up; and maintenance was in progress. The propeller was not tied and the sure step wasn't in place either. The passengers were in disarray on the busy ramp.The gate agent began an auto-board without checking with the crew; even though there was active maintenance and the dispatcher even put a delay on the flight due to weather. I walked with the passengers back to the terminal and then notified the gate agent.These types of scenarios occur frequently ever since auto-boarding was instituted. There must be some type of exchange between the crew and gate agents before boarding; on every flight. Frequently we are surprised by passengers boarding and this puts undue pressure on the crew to rush through tasks; sometimes before we can even verify whether an aircraft is airworthy and ready for passengers.