There must be a way of inviting/adding multiple users to group!
This idea has been merged with Batch access to adding users to groups in AGOL.
I suggest something around multi-editing user profiles with the same Tag (e.g. your organisation members [Company Name], external consultants [Esri UK friends]) and then adding all selected. Happy to use Ctrl key to select multiple but adding 83 members one by one seem to be ridiculously long!