They must already have a SyncNow account: an invitation to an email address that is not registered is refused, and the page says why.
There is no role to choose: everyone you invite becomes a manager. SyncNow emails them an invitation with an Accept invitation button, and the invitation also appears in their SyncNow app, where one click on Accept adds the project to their list. You never share your FTP password: the server works with the credentials already stored for the project.
Only the project owner can invite, and Managers is there only for the owner. An invitation is valid for 7 days and shows at once under Pending invitations; to follow its answer, see What happens to an invitation after I send it?. The Conversations tab appears in the project when the first manager accepts: a project with only its owner has no Conversations.