Adding to the "create a group" advice; once you add a Sim to your group, you can also click anywhere and select "go here together" to make a Sim walk somewhere you want them to.
I always use the door locks, though, to prevent these situations. Any room that would typically be off limits to guests gets locked, except for household use. Bedrooms, upstairs bathrooms if there're ones available downstairs, offices, activity rooms, etc.. Then I'll just select to unlock the door for specific Sims that I might be fine with wandering; that more often is just for anyone coming to woohoo, though. ?
As for computer use, you should have a lock option so not any random Sim can come over for a visit and start playing on it.
