You join a coop the same way you do any other company as well as the joining the cooperative normally after a probation
Poptel was also a very early ISP / ADMD (x.400) mail provider
I answered an advert in the Guardian. COOP's tend to be liberal if not left wing and attract members/workers with liberal or Left wing politics - the USA coops are not as strongly Left wing compared the the US ones.
I am using UK/EURO definitions here