Tex. Insurance Code § 463.052

(a) As a condition of engaging in the business of insurance in this state, an insurer, including a mutual assessment company, a local mutual aid association, a statewide mutual assessment company, a stipulated premium company, and a health maintenance organization authorized to engage in business in this state, shall participate as a member of the association if the insurer holds a certificate of authority to engage in a kind of insurance business in this state with respect to which this chapter provides coverage as determined under Subchapter E. The requirement to participate applies regardless of whether the insurer's certificate of authority in this state is suspended, revoked, not renewed, or voluntarily withdrawn.

(b) The following do not participate as member insurers:

(1) a fraternal benefit society;

(2) a mandatory state pooling plan;

(3) a reciprocal or interinsurance exchange;

(4) an organization which has a certificate of authority or license limited to the issuance of charitable gift annuities, as defined by this code or rules adopted by the commissioner; and

(5) an entity similar to an entity described by Subdivision (1), (2), (3), or (4).