How Do Interest Groups Form?

    One of the many questions surrounding the topic of interest groups is how in fact they actually become an interest group.  While there are many different interest groups in the political realm, there are only a few reasons attributed to their formation.
    Usually, when an interest group forms, it happens when a group of individuals share a common interest.  The only difference between interest groups and interest-based clubs is that, those who join interest groups not only feel a certain way, but they feel strongly enough to try and preserve/change legislation in Government regarding their interest.  Some groups are formed to compete for economic resources, while other groups form to show support for the preservation of rights that may seem to be dwindling.  An example of this is the National Rifle Association and its counterpart, the Brady Campaign.  Those who join the National Rifle Association do so with hopes that their constitutional right to bear arms will not be taken away.  Those who join the Brady Campaign do so to help elect Government officials who will help pass legislation to create stricter rules regarding gun use/ownage.
    Individuals who join interest groups do so because of their strong feelings towards the cause being fought over on the legislative level.  Contributing money to these interest groups is one way in which its members can feel as though they did their part in helping the cause in which they so strongly believe in.  Aside from lobbying, interest groups also rally and march hoping to spread their word, gain media coverage, and eventually, more members.

http://thisnation.com/textbook/participants-groups.html (I forgot how to cite neatly, but I know you wanted citations, I'm sorry I'll try and figure it out for next time)