How do I invite friends to my Valheim server?

The most direct route is "Join IP" from Valheim's main menu. Enter your server's address and port, which you'll find on the Network page in the panel, then the password when prompted. This works whether your server is public or private.

Your server is public by default, so it also appears in the in-game "Servers" browser under its configured name and friends can search for it there instead. If you would rather it not be listed, set Public to 0 on the Startup page; a private server will not appear in the browser.

To give trusted players admin commands in-game (spawning items, flying, etc.), add their Steam64 ID to adminlist.txt, one ID per line, then restart the server. That file is at .config/unity3d/IronGate/Valheim/adminlist.txt - the same hidden dotfile folder your world saves live in (one level up from worlds_local), not the server root, so you will not find it by clicking around from the top level. The permittedlist.txt (allow-list for a private server) and bannedlist.txt sit alongside it. If a file is not there yet, start the server once and Valheim creates it. Their Steam64 ID is found through any Steam ID lookup site using their profile URL.