Can I and my friends send together in the same envelope the tax refund declarations via DHL Express service? Who will sign receiving the envelope?
If you must mail it in, you only need to have it postmarked on the day it is due. Send it Certified Mail at the USPS for under $5, and you're set.
Do not send multiple tax documents in one envelope. This is a Bad Idea. You run significant risks that they only pick up one of the two.
Send it via e-filing if possible. This way you know they get it.
If you must send it via US Mail, send it well ahead of time (why not send it this weekend!). Then you can get an update from the IRS that they got it well ahead of the deadline.
If you must send it near the deadline, you can get it sent certified mail if you're particularly concerned; that way, if it's lost, you can prove you sent it. Nowadays that's not very expensive.
This question also has several good pointers here.
Note that a proof of mailing only asserts that you sent an envelope - nothing more. There was a court case in which the payer was fined for not sending in his return. He showed a certificate of mailing - which the IRS acknowledge that they signed for an envelope, but they denied receiving the return. He offered to help search the facility which was of course refused so he had no way to prove they received his return.
I don't know how the final judgement and appeals went. Moral - send electronic if possible and make sure you get the electronic confirmation. Else send hard copy early - make extra copies for yourself. Get a certificate of mailing to show "good faith", but don't depend on it to save you. Save your check register or credit card statement to show they billed you (that would prove they processed your return.)
Here's a link to a discussion of this issue http://www.traderstatus.com/TimelyMailed.htm. Note that there is a proposal to accept certified or registered mail as proof of timely filing.