I still find your question a little confusing.
As you know, in the assignPendingAction method there is a parameter pendingActionUsersorGroups. This parameter is a semicolon separated list of userids/groupids. Each userid/groupid is the "Principal Name" you see when looking at a user or group in EEM.
Groups themselves do not exist in Service Catalog. In Service Catalog the only way you can make use of groups is by creating them in EEM which then show up in user profiles as read-only data.
In the Service Catalog Web Services API there is the getAllUsers method in the User Web Service. There is no method for getting all the groups because, again, groups do not exist in Service Catalog.
My confusion on your query is, "Do you not know which user or group you want to assign the action?"