Use this invented text: "The neighborhood book group will meet twice next month instead of once. The additional meeting is for members who want to discuss the longer novel. Both meetings will be online, and attendance at the extra session is optional."
A concise summary is, "Next month the book group has two online meetings, including an optional extra discussion of the longer novel." It preserves the change, format, purpose, and optional status without repeating every phrase.
Keep interpretation separate
"The group is becoming more popular" is not a supported summary. The text gives no membership figures or statement about popularity. That might be your guess, but it should not appear as information from the notice.
Likewise, "Everyone must attend two meetings" reverses an essential condition. Shortening a text is useful only if the remaining wording preserves its practical meaning. Small words such as "optional," "unless," and "only" can carry important limits.
Cover the original and write your version from brief notes. Then reopen it and check every claim. Remove anything you cannot connect to the text, and restore any missing condition that changes the message. Finally, add your opinion in a separate sentence if the task asks for one. This makes the boundary between the writer's information and your response visible to the reader.