The quick way
Add a summary bot to the group once. After that, anyone who has been away types /summary and gets the last 24 hours in a single message — the topics, the decisions and the loose ends.
Been gone longer? /summary 2d or /summary 7days. Only need the last hour? /summary 1h. Want to know what one person said? /summary @name.
Catch up privately
Nobody needs to see you asking. Message the bot directly and /summary works there too — it answers about your group, to you alone, and nothing shows up in the chat.
Or don't ask at all
The bot also posts one summary every night at a time the group chooses, so the people who never type a command still know what happened today.
The honest limit
A bot can only summarize messages from after it joined — Telegram does not let bots read older history.