There’s a hidden preference that allows you to do this. If you quit Agenda first, go to the Terminal app, enter the following command followed by an Return:
defaults write ~/Library/Containers/com.momenta.agenda.macos/Data/Library/Preferences/com.momenta.agenda.macos addLinkToEventsWithInvitees -bool YES
see if now Agenda does indeed insert the link, also when there are invitees.