What you have sounds like a many to many relationship. A given account can be used by many people and some people can use many accounts.
This is typically implemented in FileMaker or other relational database by using a third table to serve as a "join" table to link the two:
People::__pkPeopleID = People_Account::_fkPeopleID
Accounts::__AccountID = People_Account::_fkAccountID
See this thread if the above notation is unfamiliar: Common Forum Relationship and Field Notations Explained
With this set up, a portal to People_Account can be placed on a People layout to list all accounts to which that person is permitted to make charges. Fields from Accounts can be added to the portal row to fill in things like the account name. In similar manner, a portal to People_Account on an Accounts layout can be used to list all People assigned to each account and fields from People can be included in that portal.
Here's a demo file illustrating the basic design set up for many to many relationships plus a few added layouts that illustrate more sophisticated approaches that are possible: