Dear rn25sgs,
1. Who is an employee ?
Sec 2 (9) of the ESI Act , 1948, defines an employee .
In a case A.P.State Electy. Board V E.S.I.Corpn,1977 Lab IC 316 (AP) mentions that the word 'employee' as defined under Section 2(9) includes casual workers also.
2. All employees to be insured - Refer Sec 38 of the ESI Act
In Bochringer Knoll V E.S.I. Corpn , 1977 Lab IC 1116 (Bom) - The definition of Section 2(9) must be given effect to for the purpose of Section 38
3. In a case : A.P.State Electy.Board V E.S.I.Corpn, 1977 Lab IC 316 (AP)
Sec 39(4) and 42(3) establish that a casual worker is entitled to payment of contribution by the employer towards employer's contribution as well as employee's contribution , though he is employed even for a day or two or a few days in a week.
From the above three points and the cases cited, i am sure that you have understood that a casual worker should be ESI insured from the date of joining and you can cite the cases in your discussions.
Once they are insured under ESI, they have to be declared in the annual returns which is to be submitted under the Factory Act 1948.
Hope this information is useful