If you are playing single player, then you can choose your map when you are selecting the settings of the game. If you are on multiplayer, the map you play is dictated by the server, so you would have to join a server with the map you want, or set up your own so you can choose which map to play.

With regard to playing with friends: The quickest and simplest way to play multiplayer would be to choose 'Host/Local' from the main menu and set up a non-dedicated session. This will open up your single player save for other people to join and play with you. If you have no other choice this option should work, however it does have a few downsides - all players will have to remain close to the host and if the host moves to another zone (like underwater or into a cave) the players will be teleported to their location. Also single player/non-ded can have a few bugs and issues with creature and resource spawning.

If you are able to do so, it would be preferable to run a locally hosted dedicated server instead. This takes more work initially, but it runs smoother, offers more freedom to players, and may be easier to manage in the longrun. To do that, check out this guide - it's the one I used back when I first started hosting myself, and even though I'm not very tech-savvy it enabled me to get things running smoothly:

https://steamcommunity.com/sharedfiles/filedetails/?id=656433788

There are also third party programs you can use to assist with hosting dedicated servers too, such as Ark Server Manager (ASM). Some say they make the process easier, but I have no experience with them so I can't say for sure on that front. Up to you if you want to research them and test them out.

On the topic of trying to join a map then getting kicked out: That could be an issue with downloading mods on the server, but if it kicks you right back to Steam not just to the main menu then it sounds like a more serious crash of some kind.

What page do you get put to on Steam? Is it a store page for DLC? If so you have to buy the DLC before you will be able to join servers running a DLC map...without buying DLC you will only be able to join the base game map (The Island) and mod maps from the Workshop.

But if you could give a little more info on the specific issue you are having, that may help diagnose the problem.

Also, I noticed you asked a few extra questions in another thread. However I'll answer them here since that other thread was another user asking for assistance with something slightly different:

Joining an unofficial server is pretty easy. First get the IP address of the server. Then go to your Steam client and on the menu at the top, select View > Game Servers. A smaller window will pop up. In that window, hit the 'Favourites' button, and to the bottom right you should see a smaller button with a + symbol on it. Click that, then paste the IP address into the text box and hit OK. That should add the server to your favourites list in-game. So boot up Ark and on the server browser, select the dropdown menu and choose 'Favourites' there. The server should show up and you can connect to it.

For info on how to join the PreAquatica Beta:

https://steamcommunity.com/sharedfiles/filedetails/?id=3561379988

And you asked how to find out what mods are being used on a server: The server advertisement in the ad section of the board should list any such mods, and third party listing sites may have an option to view the mods the server is running too. If all else fails, join the server's Discord community and ask.

Hope that helps^^

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GOD BLESS AMERICA!