In authentication sense, yes. In transmission sense, probably not.
why it's not that secure ib transmission? what can i do to increase its security?
and what shouldn't i do?
bc it's the certificate(public key) and private key that does the encryption, if everyone has the same certificate they can certainly decrypt the transmitting data.
how do they can access to other user's data? (to decrypt them then)
Intercept them on the line(on the same physical network or midpoint of transmission)