The player that is currently hosting the server is running the full version of Quake III Arena, but you only have the demo version.
In order to play the full game together, all players have to own the full game.
Purchase the full Quake III Arena to unlock all maps and other content. You can buy the game on Steam or on GOG.
In order to play the full game together, all players have to own the full game.
After downloading the full game, locate the game directory, and upload the pak0.pk3 - pak8.pk3 files from the "baseq3" directory, using the file picker below.
To find the game directory on Steam, find the game in your Steam library, right click it, then, under the "Manage" submenu, click "Browse local files".
Don't worry: even after upgrading to the full game, you will still be able to play the demo version with anyone.
For more info, see https://ioquake3.org/help/players-guide.
Alternatively, you can keep playing the demo version together. Ask the current hoster to switch to the demo version, or to let you host the game.