Some times I get invited to Google+ events and these events get shown in my Google+ calendar. The most of these invitations can be considered spam and I would like to uncircle the sender in such cases, but I can't find the information on who has invited me (looks like this is not always the same person as the event administrator). How to know who was the one to invite me?
When I receive an invitation in my Google Calendar (I assume you mean Google Calendar, not Google+ Calendar), the sender of the invitation will appear when I click on the event in question. It is mentioned just below the 'Where' entry.
Unfortunately, I'm not sure if this is also the case with invites received from Google+.