given a list of type User or Group, is there a way to iterate through that list

It can be achieved by using fn!runtimetypeof(ri!userGroup). It returns an integer for type of given data.
4 - User
5- Group.
Use apply function on the list with a rule which calls this function.

User below rule
Rule name:-APP_isGroup
Input:-group_UG(User or Group)
=regexmatch("Group:(\\d)*",tostring(ri!group_UG))

This rule will return true if input of Group type.