And is there any difference between "Look, this is our tour guide" and "Look, it's our tour guide"?Are these interchangeable in this situation without any change of meaning?
Yes, there's a difference. Saying 'It's our tour guide' sort of implies suprise, or unexpectedness, as though he/she appeared beside you unexpectedly; 'Look, it's our tour guide' implies that one speaker sees the tour guide from afar and is making the other aware of his/her presence.
'Look, this is our tour guide' sounds like an emphasis, and 'look' is being used to mean 'listen'. It makes me think of two people arguing about who is their tour guide, while looking at pictures of a variety of tour guides (because of the 'this', which requires the thing to be close to the speaker). As in:
Speaker A: I'm sure it's this one (points to picture 1)
Speaker B: Look, this is our tour guide (points to picture 2). I saw her when I booked the tour!
You can't say 'this' without some sort of gesture; poiting, nodding etc. To say 'this is our tour guide' in any situation you have to be showing her, either as an introduction, or poiting from afar (in which case it would be 'that'). If you say 'look, this is our tour guide', the 'this' implies he/she is there (because 'this' means 'the thing near me') and saying 'look' would be to either direct the attention fo the listener, or to make him listen to you. Either way, it is rude! The tour guide is there, talk about her like she is there!
Basically, unless it is an introduction, you don't refer to someone who is present among the speakers/listeners as 'this' unless you wish to be rude. And if you are introducing someone, you don't start with something like 'look'.