Players automatically added?

So I just put up a campaign listing. Had one player look at it so far, and post in the discussion forum. And, voila, he's added as a player. Didn't invite him. He has no idea how it happened, either. Bug or feature? Did I miss a setting some where?

did you include the join link in your OP? If you did, anyone and everyone can join until you boot someone, thus changing the join link automatically and making the previous void.

I see.. Very helpful, thank you.

Gary answered that well. It is recommended not to post your Join link publicly. You can enable the Looking For Group listing for the campaign (the Link to Enable LFG is at the top of your Campaign Details page). Then share the LFG link on public forums instead. People can look at your LFG and post their interest/application there. It will give the GM a blue button to "Invite Player" so you can choose who to send the actual joining link. Or, if you don't want to post an LFG listing, just go ahead and put the details in the forum and ask the applicants to reply or private-message you for the Join link, and you can message it to them.