Hi Alex,
In order to register as a company, your group must be an actual company.
In both registration type (individual and company), only 1 Microsoft Account (ex:
example@outlook.com) will be able to access the Windows Phone Dev Center account. You will need to share the credentials for the Microsoft Account if you wish multiple members to access the account.
Shared Windows Phone Dev Center account with multiple Microsoft Account associated with it in currently not a supported scenario.
Thanks
David