there in a fn-IsMemberOfGroup() inline function.

it's not directly available in rule's formula builder, however, despite of that it is usable and working there.

Member of Group and Formatting Rules

Thank you Marian. it worked for me.

I have a rule that enables/disables the controls depending on the user mentioned in the people picker control on the form. Below is the rule. If i include more than 1 user it obviously fails. Is there any function i can use to get the job done?

not(userEquals(Current User,PIA)) && not(userEquals(Current User,FinancialAnalyst)) && not(userEquals(Current User,POA)) 

unfortunately nintex doesn't provide methods/ways how get or validate single entry/option out of multivalue controls or fields.