Legally Required Participants

From IDEA - Sec. 300.321 IEP Team.  See Ed.gov - IEP Team

(a) General. The public agency must ensure that the IEP Team for each child with a disability includes--

(1) The parents of the child;
(2) Not less than one regular education teacher of the child (if the child is, or may be, participating in the regular education environment); 
(3) Not less than one special education teacher of the child, or where appropriate, not less then one special education provider of the child;
(4) A representative of the public agency (LEA) who--

i) Is qualified to provide, or supervise the provision of, specially designed instruction to meet the unique needs of children with disabilities;
(ii) Is knowledgeable about the general education curriculum; and
(iii) Is knowledgeable about the availability of resources of the public agency.

(5) An individual who can interpret the instructional implications of evaluation results, who may be a member of the team described in paragraphs (a)(2) through (a)(6) of this section;
(6) At the discretion of the parent or the agency, other individuals who have knowledge or special expertise regarding the child, including related services personnel as appropriate; and
(7) Whenever appropriate, the child with a disability.

(b) Transition services participants.