This seems to be a Question every scrim, so i thought id take a screenshot. It's pretty simple. Click join game, when the server list comes up, Click on password circled in red, type in the password, click ok, then join the server.
1v1 Match - PW gets posted in the Krew Members section only. Usually to avoid non members coming in spoiling the match
Recruit's Scrim - PW gets posted in the Recruits area. We do make exceptions if a non-member/recruit would like to join. We usually let them know they are only able to spec because the Scrim is only for Members and Recruits.
Those are the only reasons I can think of that I can see why a server would get locked.