Collaboration is essential for any calendars and you and you can invite anyone using an email to your Atomcal events, doesn't matter if they are an Atomcal member of not.
Simply start by pressing the + icon next to where all the guests are shown. Then type in the email or if you want to search and invite an already existing Atomcal member then you can type @ followed by their name of email and then select the member you want to invite and voila.
Event guests can also be invited in the same way using @ in the event notes section and this will add this invited user to the guests list.