I want to audit who gets added to the Account Team.
Account Team is very limited. It is not like Opportunity Team where you can have custom page layouts and fields. Account Teams can not even have triggered associated with them. The options that I see are: 1) Create a custom "Audit" object for teams and a scheduled Apex job to query for newly created Account Team records and create the Audit records. 2) Create a custom coverage team concept that synchronizes with the standard team. We have done this before, but it is VERY complicated to maintain.
You can use SOQL to query AccountTeamMembers. There are many tools that you can use to execute SOQL including developer console and workbench.
Here is a sample SOQL query:
select User.Name, Account.Name, AccountAccessLevel, CreatedDate, CreatedBy.Name from AccountTeamMember