In my experience it should just work. How are you launching the server?

o i think he means how to made a friends only match on a custom map in l4d2, cuz the official dedicated servers couldnt run workshop campaigns for the longest time so players would try to make a friends only match and it would randomly select 1 of the steam group servers being hosted by private groups, which if the group set their server to public then players in the group could still join the server through the main menu / server browser even if the lobby was set to friends only.

unfortunately theres not much to fix that, they did make official dedicated servers able to run workshop campaigns now since the end of 2016 but theres also a lot less of them now, so you might run a default l4d2 campaign and still will have to use a steam group server, which im sure is annoying for players that want regular l4d2 even though the groups running the servers do so for free and without them nobody would play online custom maps very much.

you can try making a lobby and changing the server type from official to local host, then the players connect to the lobby leaders pc and his set up runs the match, you have to use sv_consistency 0 command in the console tho or it crashes from players using different mods. It might still crash tho cuz the local match is just buggy in l4d2 unfortunately.

your best bet is to probably find a steam group that runs a regular default 4 player l4d2 server and join the group so it shows up for you when making a steam group server lobby, so at least you can pick the server you want that has the gameplay you want, but if the clan sets it to public then anyone in the group can still join.

or if he does mean literally running a l4d2 dedicated game server thats private for him and his friends then you would have to install a dedicated server like usual, then take and make a invite only steam group that will be just for you and your friends you want to join your server.

then in your server.cfg file for your l4d2 server you need the line sv_steamgroup 0000000 (the number you get if you go in your steam group and click edit group info) and also the line sv_steamgroup_exclusive 1 so that way only steam group members of the group you connected to your server can start a match on your server when its empty and idle.

that way nobody other than group members can start a match on it and if you guys pick friends only in the lobby its as good as you'll get to a completely private server, other than putting a password on it in your server.cfg file and telling your friends what it is. That is if he does mean an actual server.

o and to put custom maps on your dedicated server just subscribe to the map and find it in your workshop folder in your own games l4d2 files and copy and paste the vpks to the addons folder in your server on your server box, whether they are from gamemaps so an addon vpk or the workshop they go in the addons folder for a server.

i use evolve

*Walter ran away from this chat thing*

Shut up