Okay, here’s my write-up on adding companions in Genshin Impact’s teapot, just like I experienced it:

Alright, so I finally got around to messing with the Serenitea Pot again, and this time, I wanted to populate the place with some of my favorite characters. I’d heard it was possible, but hadn’t really looked into it until now.
First thing I did was chat with Tubby. You gotta do that, ’cause Tubby is basically the teapot’s manager. After the chat, I opened up the build menu. It’s the same one you use to place furniture and stuff. I was kinda expecting some convoluted process, but nope, right there in the menu was a new option: “Companions”.
I clicked on “Companions” and was greeted with a list of all the characters I own. Sweet! I chose the first character I wanted to add (it was Xiangling, because who doesn’t love Xiangling?), and then the game let me pick a spot to place her. You know, like placing a chair or a tree.
I fiddled around with her location for a bit, making sure she wasn’t clipping into anything or looking totally out of place. Then, I just saved my changes. And boom, Xiangling was chilling in my teapot! I repeated this process for a few more characters – I think I added Noelle and Xingqiu next.
One thing I did stumble on a little bit was letting friends visit to see my characters. I opened the map while I was in the teapot, and in the top right corner there’s a button about guests. You gotta use the dropdown menu in the new window to set it to “Always Allow Direct Entry” (or something similar, I can’t remember the exact wording). This way, even if I’m offline, my friends can pop in and admire my teapot decor (and my character choices, of course!).
Also, I learned a little trick for quick setup. If you’re on PC, you can hit the Triangle button to bring up the menu to select the people you want. On the Party Setup Screen, you can use the menu that appears by pressing Up to Create or Disband Teams.
I also remembered needing to do that World Quest called “Idle Teapot Talk” before I could actually invite anyone. Make sure you do that quest first!
Overall, adding companions to the teapot was way easier than I expected. Now my little realm feels a lot more lively!
