Those would be called inquiries and yes they can lower your credit score. Multiple inquiries in one year can be looked upon as desperate for money. Especially if your credit is being checked by credit cards. And the average is about 5-15 pts.
If you co-sign with someone with bad credit it WONT lower your score. BUT, if that person lets-say, misses a payment on whatever you co-signed them with then YES it will negatively affect your score.

You can learn more credit education on

=] hope that helps!