Tap the share button on the episode's details page, or next to the title in the expanded player.
On a phone, this opens your phone's share sheet. In a browser, it uses your browser's share option, or copies the link if there isn't one (you'll see Link copied).
The link opens a public page for the episode, so the person you send it to can listen even without a Castria account. Links always start at the beginning of the episode.
You can't share a whole show yet.