Somebody know what happens when you add an Item Group to a transaction (server side)? the lines that expand(in the UI), the members are immediately available and the line count would increase or you would have to detect if the line being added is a group and manually adding the group members so you have control over each line/member of the group being added to the transaction?

Yes, they are automatically added right after you add the main group item (in dynamic mode), or when the record is saved (in standard mode)