We just upgraded to the new Mac 5 and we don't see the conference button anywhere.
In the call window, click the Add Participant button.
In the Contact Picker, type the name of the person you wish to add, and then click on their name when it is displayed in the list. This adds them to the list of new participants. Repeat for everyone you want to add to the call.
After you have finished selecting people, click Add or press Enter on your keyboard to call the newly added people.