As I know, they are randomly available only, depending on the stats of your chars. "Weak bladders" are more often on the toilets, "Clean freaks" shower often and so on. You can use items to "force" them to go to toilet / shower:
In your room, click "Leave room" then "Meetup". On the meetup menu, choose the character you want to go to toilet / shower, click on the "item" icon, choose "Diuretic" (first) for toilet or "Sudorific" (second) for shower. Then, go back to your room and "Sleep".
After this, go to "Meetup" again, choose the character you gave the item and "initiate fun". Now the map selection is reduced to one of the toilets (randomly western or japanese style) or the shower, depending which item you gave.
Correct me, if I'm wrong or there's an easier way to do this