When a Prospect is initially created, ownership is automatically assigned to the user creating the Prospect. However, the Prospect can be assigned to another user within the organization, either during the initial creation or when editing the Prospect record.
Emailing Prospects Owned by Other Users
The Admin on the account has the ability to set whether users can email other users prospects. By default, users with the Default profile can only email prospects that are owned by them.
Under Settings > Profiles, your Admin can turn this setting off or on for any particular profile. This means that users will be able to email their reports prospects, or all prospects, depending on the setting.
Under the Heading Prospect Engagement you'll see that drop down and you'll have 3 settings.
All Records - Any user can email any prospect regardless of owner
Owned and Reports' Records - User will only be able to email owned prospects and their subordinates prospects.
Owed Records -Will only be able to email the individuals prospects that they own.