CiviRules is not tagging users or adding them to groups
The first thing that strikes me is the trigger is "participant is added". That means it is only done for new participants, not for existing ones. Should it not also do this for changes to participants?
Personally I have not done anything yet with the event participant trigger, so I would focus on testing that one first.

I do regularly use tagging of contacts and adding to group actions, so I know that these actually do work, the problem lies elsewhere. Next week we have planned a small CiviRules sprint, I will have a go with these examples.

Okay, here are my test results.

Test 1:
This is the rule I have entered (and apologies for the Dutch in some places):


If I now register a contact for the event and immediately give them the status "attended" this works like a charm:




Obviously this will NOT work when I register a participant with the status "registered" and then later this is changed to status "attended" as the participant already exists, so the "participant is added" is not triggered. To deal with this I have a second rule on "participant is changed":


Again, this changed rule works like a charm.