Yes, you need to sign in after restarting the PC.
Yes, if someone can sign in, (s)he knows the password for his/her account. AxCrypt accounts are intended to be personal, with personal passwords. So if you have two employees Alice and Bob, they have different accounts and different passwords as do you. No-one can open each others files, unless keys are explicitly shared.
You can try all this out, and also read the documentation and view the videos at https://www.axcrypt.net/ .