Usually the logo designer focuses on the image for the logo design. That portrays 90% of the association's purpose. The text therefore should be kept to a minimum. It should be mainly just the association's name. You don't really need a tagline, or a slogan for an association design. However, if you would still like to add it in, make sure it is not more than four or five words placed circularly around or under the image. You would want to give the image more stand-out space, rather than the text.
Clearly understanding the demands of association logos will help professional designers in handling such projects with confidence.