The one-command way
In the group, type /summary for the last day, or give it a window: /summary 3h, /summary 2d. In a group with topics it summarizes the topic you're in; add all for the whole group, or @name for one person.
Rather not ask in front of everyone? Message the bot privately and send the same command — it answers about your group, just to you.
Or have it done for you every night
Once a day, at the time you pick, it posts a short summary of the day in the group: what was discussed, what was decided, and what's still open. A plain factual summary by default; a roast if your group wants one.
And for anything older, /ask answers from the whole archive — when did we agree on the rota?
Setting it up
Quickest: use the KeeperBots bot. Create your account, pick it, and tap the button that adds it to your group — no bot to make, no token to paste, and privacy mode is already off.
Or create your own bot with @BotFather, turn its privacy mode off, and paste its token into KeeperBots.com. The setup checks each step with Telegram and finds your group for you.
What it can't do
It works in groups it has been added to. It can't read your private chats with other people — no bot can.
Telegram gives bots no message history, so it summarizes what is said after you add it, not what came before.
Can I summarize a chat I'm only a member of?+
Someone who can add members has to add the bot. If that isn't you, send them the link.
Do the other members see my /summary?+
Only if you ask in the group. Asked privately, the answer goes to you alone.
Is there a free trial?+
Yes, 7 days with every feature and no card. The days start when the bot joins your group.