How does the fifth amendment play into that?
Pleading or Taking your fifth amendment rights in the US means basically you are not going to answer their question on the grounds that answering that question forces you to incriminate yourself. You don't have to help prosecutors lock you up.
You can't take the fifth over questions of other people, but you can if by answering you would be admitting you were committing a crime as well.
Example: You and your friend, Mike, work as salespeople at a high-end TV store and during the night you see Mike take money but not ring up a sale correctly, then pocket the money. You before you report this to your manager, Police walk into the store and arrest Mike. The manager had been suspecting someone was stealing but didn't know which employee it was.
Now you were on the stand as a witness and the lawyer asked you "While were at your job, did you ever see your friend, Mike, a fellow salesman, steal the cash from the till, or ring up a sale but pocket the money".
Well in this example you have to answer (how you answer is up to you...you shouldn't lie as a felony isn't worth it if anyone found out you had lied). You can't take the fifth as answering their question doesn't incriminate yourself.
A good example comes from wikipedia:
One famous example of "taking the fifth" in recent history was when Colonel Oliver North was asked to testify before Congress regarding his role in destroying documents during the Iran-Contra Affair. North refused to answer on the grounds that his answers would incriminate him for obstruction of justice.
In Matt's example, IF they ever took him to court, he has the right to take the fifth if they ask him a question like "Did you, at any time, sell lasers rated over 5mW to person or persons inside the United States of America?", they could have the evidence of sales and even his posts here but he doesn't have to admit his guilt. Pleading the Fifth is like pleading "no contest". You are not admitting guilt nor denying it.
Now I am NOT advising anyone here...just laying out examples.