I think , under any circumstances its extrmely important to speak to the host , if you are intending to take anyone along .... otherwise both the host and who ever you have taken along with you , will feel out of place and uncomfortable ...
I agree whole heartedly.
If at last minute someone shows at my place I will call the host and let them know that we cannot come, because , and then we will let them know that we have company. If they would accept the excuse and would not invite the extra person with us then we do not go.
If I have guests at my home and I am invited I would let the inviter that we have so and so at our home , therefore we will not be able to come , if our guest are also invited then we will take them along and go .
Never in my life, ever, anybody told us not to bring our guests, we were telling them about, though. These are our desi traditions and customs that somebody's guest is our guest too.
At my place I welcome anybody who shows up uninvited with anybody else and I make sure that they feel comfortable and welcomed once they are at my place.