This is my first reply post, so I hope I get it right!
A- I've accepted an invitation to Terry's house for dinner.
You use the present perfect to show that something has happened in the past but is unfinished, or to show when a past action is relevant now. So you accepted the invite (past), but the sentence sounds like you haven't yet had dinner at Terry's house, so this is an unfinished action.
B- I received an invitation to Jannet's party,I can't wait to go!
I think the sentence should be broken up into 2, i.e. 'I received an invitation to Janet's party. I can't wait to go!' This makes it easier to explain why you need to use the past simple. 'I received an invitation to Janet's party.' is then just about what has happened in the past, the action has happened, it is finished.
Hope that was helpful.