Do either Comey or Rogers have the authority to DECLINE the invitation from Nunes?[/size]
Interesting question. They can decline, as they don't work for Congress and haven't been subpoenaed. But on this it would be a very bad move.
Both of them serve at the pleasure of the president, and refusing to meet with Nunes about an issue that directly affects the president would make them vulnerable to firing.
Handled properly, their failure to come clean on apparent crimes can be made into a reasonable excuse for firing them. (OK, this is Trump -- it wouldn't be handled properly, but still....)
If neither of them is actually complicit, their best interests would be served by cooperating to a certain extent. If either of them knew and looked the other way, their bests interests are served by showing up and being unable to recall....
Smart political strategy would have Nunes and Trump working together; but I suspect Nunes is acting on his own.