I really think you need to get a behaviourist in to assess the situation.
It could be that she didn't learn bite inhibition correctly as a pup and doesn't realise what she is doing is hurting, but it does sound like she is reacting out of fear... the situations you have described are all times when she would have high stress (someone new arriving suddenly, play etc) so you really need someone professional to see what it is that is stressing her and then advise on dealing with the behaviour.
I wouldn't recommend you shout and point, as this will only increase her stress (and she won't necessarily know why you are shouting). If she does start barking or nip I would just get everyone to quietly and calmly walk away (until you have more appropriate advice from a behaviourist). I would also avoid pushing her into stressful situations, so keep play very controlled (especially with children about), don't approach her too fast, try putting her into a different room when visitors arrive.
Make sure your behaviourist is properly registered and uses positive methods (if she is nervous, aversive methods could make matters so much worse in the long term). Good luck!
