A group of insurers organized to provide workers compensation coverage for a particular type of risk or size of exposure, when there are difficulties in obtaining coverage in the voluntary market, and which share in the profits and losses associated with the program. Certain insurance companies issue policies and handle claims, but the ultimate costs are borne by all companies writing insurance in that state. In most states the market of last resort is referred to as the assigned risk plan however some states use the JUA type organization.
For example, as found on the MJUA website, the Minnesota Joint Underwriting Association (MJUA) was created in 1986 by the Minnesota State Legislature. This association provides liability insurance coverage only to persons or entities who cannot obtain insurance through normal outlets or standard markets. When they are unable to secure coverage and the insurance is required by statue, ordinance or otherwise required by law then the person or entity who needs insurance can apply to the Joint Underwriting Association.
From a purely workers compensation stand point this term is normally not used. The workers comp term that is nearest associated with JUA would be the Assigned Risk Plan or Pool. In regards workers compensation, it's this mechanism in most states that is the market of last resort and where applicants who may not be able to secure coverage through standard markets are able to find and but a workers compensation policy.