If the group call is already happening, any other internal user can be added via Participants tab, the Moderator can add users to call instantly and they can also join the call.
If the group call needs any external users, that user can join by using the link which may be accessed as shown in figure. If you want to configure the link with a password, check Password protection and enter the password, then share the link to external user. Users will get a page with a field to enter password, he can enter the password and can join the call after entering the password..
Fig. External link share option