When talking "shared" are you talking about users all sharing the bandwidth on a PON? Or where a strand of fibre happens to share the same physical space with other strands inside a cable?
It is highly unlikely, nay impossible, to start sharing the PON or bandwidth from the aggregation node up to the exchange... that's just the point at which the one strand of fibre from the splitter devices gets spliced into the fibre spine. Your PON's strand will be sharing cable with the strands serving other PONs.
All of your sharing of bandwidth happens within the splitter device - 32 strands of fibre at one side (to 32 individual properties) meeting 1 strand of fibre on the other side (to the exchange, through the fibre spine). This splitter device defines the PON, and is found somewhere between the AG node and your house.
The layout can be seen at this point in this Openreach video:
The 32 properties do not have to be on the same DP, or installed at the same time.
Yes, you are likely to be the only user connected to the splitter node at first, so getting the full bandwidth uncontended. But, in time, 31 others will join you.