Cycling Question (Probably for "Six" to answer)

[Oski]

Can anyone explain this to me? This excerpt was from a news story:
[ QUOTE ]
The Italian was trying to catch a group of six riders who, in an effort to win, pulled away from the main pack early in the stage through eastern France.

Armstrong went tearing after him. When he and Simeoni caught the escape group, the riders there told Simeoni he was not welcome. They knew that their chances of winning the stage were nil if Armstrong stayed with them. Simeoni eventually demurred, breaking off his attack and returning to the main pack -- with Armstrong.

``Armstrong demonstrated to the entire world what type of person he is,'' Simeoni said. ``It is not reasonable that a great champion doesn't give a chance to a small rider like me and the others. ... I suffered an injustice from him while everyone was watching.''

Juan Miguel Mercado, who was part of the escape group Simeoni wanted to join, went on to win, beating fellow Spaniard Vicente Garcia Acosta in a sprint at the finish at Lons-Le-Saunier.

[/ QUOTE ]

I know the underlying problem between the two riders, but I am curious as to this racing dynamic of one cyclist able to "reel in" another. Also, what is this business about "the riders there told Simeoni he was not welcome." What does that mean? Do you have to cycle within your own group or something?

P.S. I realize I know so little about this sport that it was foolish for me to bet on it this year (I bet even money L.A. would lose).