The Colorado Society of Enrolled Agents (CoSEA) is an affiliate organization of the National Association of Enrolled Agents (NAEA). As such, a prerequisite of becoming a member of CoSEA is membership in the NAEA. Essentially you will be joining both at the same time.
The NAEA website has an overview page that describes the various categories. Briefly, these include the regular membership with the requirement to be an Enrolled Agent. In addition there are associate memberships for Non-Enrolled Agents who work in the field of tax, academic associates for students and professors in tax, accounting or finance, and an emeritus membership for retired NAEA members.
Join NAEA and CoSEA: Click on the link to go to the NAEA overview. There you will have a couple of choices: click the membership type that applies to you and create a printable application or, near the top of the page, follow instructions for creating an account and then joining online.
Either way, when you select a state affiliate, select Colorado, complete submit the application.