When details are entered into a Booking Form as part of a completed Order, a Member Record is created for that person. If they have an Email Address, then the records are linked together by this data point.
Depending on who the Member is in regards to the Order (Purchaser, Participant, etc), they are then assigned to one of the Security Groups which are set up within the BookingLive system to regulate what access they have, and how they interact with the system.
By default, the Purchaser group has access to the MyAccount, and the Participant group does not.
As such, anyone who is added to an Order as a Participant will not be able to access the MyAccount. However, if they complete an Order as a Purchaser, they they will be able to from that point onwards, unless they are removed from the Purchaser Group.