Many Guiding official history books claim that the first official Scout Rally was held in September 1909 at Crystal Palace in London, attended by 11,000 Boy Scouts. They also claim that to Baden-Powell's surprise, there was a small group of girls dressed as if they were Scouts, who had gatecrashed the event - his surprise because he hadn't known there was such a thing as girls in Scouts. It's also often said that after they had begged him for 'something for the girls' he agreed to think about it.
There are several points of this story that are untrue.
1) Crystal Palace was not the first Scout rally, there was one held in summer 1909 at Scotstoun Stadium, Glasgow.
2) It was attended by 11,000 Scouts, including a number of Girl Scouts from different Troops. (Some writers suggest several hundred Girl Scouts).
3) Baden-Powell was not surprised to see Girl Scouts there, as several had openly applied for tickets. This was not unexpected, because several had written to him to ask about joining Scouts during 1908 and 1909, and he had sent enthusiastic handwritten replies to them assuring they were welcome to register as Girl Scouts, and could wear any badges they earned. He had also written about Girl Scouts in his personal column in "The Scout" magazine in January 1909.
4) They weren't dressed 'as if they were Scouts', they were fully registered Girl Scouts, and thus entitled to be wearing the uniform for Girl Scouts as specified in the 1909 edition of Scouting for Boys (yes, it specified the correct uniform for Girl Scouts). By that time there were over 3000 openly registered Girl Scouts; there may have been many more either not registered, or registered under initials, or names which did not clearly indicate gender.
5) There were several Troops and Patrols of Girl Scouts there, as well as individual Girl Scouts, of which only one Troop had gatecrashed - namely the Troop from Peckham Rye. And they only gatecrashed because they decided to go at the last minute despite the fact they hadn't applied for tickets to attend a ticketed event. Had they applied for tickets, they would have received them. One of the reasons the gatecrashing succeeded was because, not having enough money for tram fares, they opted to walk, and due to poor planning they arrived at the venue an hour late, by which time the event was already well in progress, and thus the entrance was lightly staffed. Plus, they gathered round the corner in order to march through the gates in formation at full speed - anyone who tried to stop them would have been risking injury.
6) There were various clubs for girls already operating in that era, including the national organisations Girls' Brigade/Girls Guildry, and Band of Hope. So girls didn't need 'something for the girls' - they already had something. They just wanted a more lively and adventurous programme of activities than these existing groups offered.
Given this, the way to get a clear idea of what actually happened is to look back at contemporary or near-contemporary accounts, to see what was said about it at the time, or shortly afterwards, when memories would still be fresh - either accounts by participants who were actually there, or views given by newspaper correspondents, who can be considered to be independent witnesses of events. Several of the vintage history books (including the first one in 1932) were written by people who were not there.
April 1914 Girl Guides' Gazette - a short history of the Pinkney's Green Girl Guides, founded on February 6th 1909 as the 'Girls' Emergency Corps'.
"I must pass over the doings of that first summer, and briefly record our part in that very first great Scout Rally at the Crystal Palace in September, 1909 - an event never to be forgotten by those who were present to listen to, to cheer, and to see for perhaps the first time their great founder, the originator of this boundless movement - the Chief Scout!
Yes, there were thrills, many, that day! Who forgets the competitions, the rain that spoiled the fire-lighting; the eight silver-mounted staves awarded to the Pinkney's Green Scouts! the displays in the vast arena, the Scouts seated around the sloping track, the order to "fall-in" and march into the Palace (how Troops took stock of one another!); the marshaling of those eleven thousand Scouts; the cheers from those eleven thousand throats that would not be hushed; the hats on staves; the message from H.M King Edward, read by the Chief Scout! And then the march past - but what has this to do with Guides? why, three of us were there! Yes the P.G. Scouts, led by the tallest, marched past in their allotted places, with the two girls selected to represent the Girls' Emergency Corps as a special privilege (for was this not a Scout Rally?), in line with myself, bring up the rear.
The girls seemed to get a special cheer as they passed the saluting-base. There were a few other girls there, quite an imposing number in one corps alone.
Evening brought the 'fire-works', viewed from the terrace, and the journey home; not the least exciting venture of the day being the ride between Paddington and the Crystal Palace - both ways - in a motor van, through the kindness of Messrs. Gamage."
The Times Newspaper - 6th September 1909.
"The first annual rally of Boy Scouts took place at the Crystal Palace on Saturday, and the building and grounds on the occasion very much resembled a large camping area. There were no fewer than 11,000 Scouts present from all parts of Great Britain, and among them was a troop of girl scouts who excited considerable curiosity."
"General Baden-Powell took his stand on a platform below the great organ during the march-past, which occupied three-quarters of an hour. The girl scouts were loudly cheered as they passed, and so too were the Scottish Scouts, who were headed by four kilted boy pipers."
The Scotsman Newspaper - 6th September 1909
"The big rally of Boy Scouts at the Crystal Palace on Saturday provided eloquent testimony to the growth of this relatively new movement. Boys arrayed in all kinds of uniform came to the Sydenham Pleasure Grounds from all parts of England, Scotland and Wales with slouch hats, shirts of khaki, green or blue and the multi-hued scarves characteristic of the various corps. They looked picturesque in the extreme. The majority had an equipment of staves, belts, water bottles, haversacks and knives.
One of the most interesting features was provided by the Girl Scouts who, in emulation of their brothers have commenced operations on similar lines in various parts of the country. Prominent among these were the members of the 2nd Reigate Troop, under the command of a girl sergeant. They attracted much attention in their Scout's headgear, light blouses, and serge skirts, with haversacks slung over their shoulders."