Quick (hopefully) question. I need to add a specific user to a public group. Ids for both are listed within an object called Fund Document Entitlments, so all I have to do is use those two ids, but I'm not sure how. Here's my code so far:

public static void AddUserToPublicGroup(List<Fund_Document_Entitlement__c> items){
	Set<Id> portalIds = new Set<Id>();
	Set<Id> publicGroupIds = new Set<Id>();
	for(Fund_Document_Entitlement__c fde : items){
		portalIds.add(fde.Customer_Portal_User_ID__c);
		publicGroupIds.add(fde.Public_Group_ID__c);
	}
		
	List<Group> pgList = [select Id from Group where Id in: publicGroupIds];
	List<User> uList = [select Id from User where Id in: portalIds];
		
	for(Fund_Document_Entitlement__c fde : items){
		for(Group pgroup : pgList){
			for(User u : uList){
				if(fde.Customer_Portal_User_ID__c == u.Id && fde.Public_Group_ID__c == pgroup.Id){
					//what do I put here
				}
			}
		}
	}
}

 Somehow I need to say "add the user with user Id u.Id into group pgroup.Id. How do I do that? Thanks!