Let's say, I've joined TELEGRAM group...
I am just a typical member of GROUP (and thus, cant use any bots there.. ?) so, I am unable to find out the way, how to get GROUP ID.
via code:
If you are a member of a group, you should receive updates from that group when ever there is any activity from that group. The updates will contain a chats list-element from which you can get a Channel which has the following relevant fields:
id: group_id,
title: "the_group_title",
username: "group_username"