For a lot of smaller companies (not only webhosting companies) it is important to unite yourself. Together you stand stronger and you can make a fist to large organizations. For example: registries.
In Holland we have a few hosting associations. The largest one is the DHPA (Dutch Hosting provider Association) But only very large companies like KPN and Wide XS are member. Besides the DHPA there are a few smaller associations. I am member of IspConnect. It is a small but good webhosting association. You may pick out an association which you feel comfortable with. But please keep the following things in mind:
An association with a very small contribution (less than 100 euro) is probably not a serious association. How could you organize members days and other activities with such a small amount of money?
Look at the other members. Are you familiar with the members? Are the members serious companies? Remember that you will be seen as one with the other members…
Is the association “active”. Do some search in Google. Does the organization appear in the media a lot? Do news websites take the association seriously?
I guess there are plenty of guidelines to think about. But the most important one: Look for an association which speaks your language! You have to feel comfortable with it!