What associations do instructional designers join?

In no particular order...

• Association for Educational Communications & Technology (AECT) - This would be my pick for the most scholarly association. For example: They periodically put out giant handbooks of instructional technology and design plus they publish the top-tier tech & design journal, ETR&D. 
• Association for the Advancement of Computing in Education (AACE) - Another group of scholars...but not as rigorous as AECT. AACE is an umbrella for several organizations, all of which are at least peripherally interested in ID. Publishes quite a few journals.
• International Society for Technology in Education (ISTE) - Probably the best-known, certainly the largest. More K-12 focused / practitioner-focused than other organizations here. Famous for their standards documents and their massive, massive annual conference.
• Immersive Learning Research Network (iLRN) - If you're interested in VR, pay attention to these folks...
• International Society for Performance Improvement (ISPI) 
• Association for Talent Development (ATD)
• Training Magazine - Not at all a scholarly resource, but it gives you a sense of (one part) of the ID community -- looks like corporate / consulting / workforce development.