When you book your first ride, the ride leader will get in touch with you to introduce themself. If you need to borrow a bike, this is a good time to check whether they have bikes to borrow for the ride.
The ride leader might ask if you want to be added to the group WhatsApp group. This is a good way to get a feel of what everyone is like, but you don't have to join if you don't want to!
On the day of the ride, check your bike is working before you leave the house. Try to arrive at the starting point a bit early in case you get lost, as they're sometimes difficult to find.
When everyone is together, the ride leader will introduce you to the group and make sure you have someone to ride with.