Open the session and tap Invite a friend, just under the booking buttons. On a phone that opens your usual share sheet; on a computer it copies the link. Send it however you like.
Your friend does not need an account to look. The link opens a preview - the title, who is hosting, the time in their timezone, how long it runs and how many people are going - with a line saying you invited them. If they want in, Join free takes them to sign up, and when they are done they land back on that same session rather than the general schedule. If they already have an account, they log in and arrive there too.
The preview shows only what is public about a session. It never shows the agenda, the chat, or the names of the people who have booked.