For example, what is the subject of the following sentence?
The speaker or the opponent?
Hope I've understood you correctly:
くださる means 'to give'.
(You) give (me) the favour of making sure to come at 8 o'clock. = Please make sure to come at 8 o'clock
The person doing the giving (you) is the opponent which is the subject of くださる. The speaker (me) is the indirect object.