As of 1.8, there aren't many people you can get in the studio alongside Anna.
- Josef, if you tell him you and Anna have been doing Starmaker
- Charlotte if you chose to go the Anna route
Might be more people (Ken maybe, I dunno.), but those are the only ones I've found thus far in 1.8.
For Ken - Once you have 4 hearts with Anna, you can invite Ken to the basement if you go the Anna Share Route.
There is supposed to be one more studio invite coming in 1.9 (to be confirmed who)
Supposedly you can also use the "Gallery 1.1.4 @Mr ModMan" mod to place characters in any location you wish, but never used it....
Besides that you will have to use Stable Diffusion/Photoshop and create your own