You are not obligated to participate if your company sets up an ESOP and offers you options, unless your employment letter suggests otherwise.
It's worth noting, however, that participating in a standard call options scheme comes with no financial commitment from the option holder unless you decide to exercise your options. Exercising is optional, which means you have time to weigh up the decision while still being part of the scheme.
Additionally, if the company is not successful, and options have not been exercised, then there is no loss to employees participating in the company ESOP.