My visiting friends or family members are of course welcome to use basically anything I own, including my computer if they need to. I rarely have visitors, but the few I get I am very close with (siblings/BFF), so I have no problems with sharing my stuff. My house is theirs, basically.
Of course that goes for my sims friends too. I don't let them invite strangers/aquaintances to come inside their home (I'm looking at YOU, random Saturday afternoon visitor). I lock the doors to any room with a computer, but otherwise friends/family can do whatever.