Signing and encrypting of messages is possible using emacs-mime (See (emacs-mime)Composing), most easily accessed through the Attachments-menu while composing a message, or with M-x mml-secure-message-encrypt-pgp, M-x mml-secure-message-sign-pgp.
The support for encryption and signing is independent of the support for their counterparts, decrypting and signature verification (as discussed in MSGV Crypto). Even if your mu4e does not have support for the latter two, you can still sign/encrypt messages.
Currently, decryption and signature verification only works for PGP/MIME; inline-PGP and S/MIME are not supported.
Important note: the messages are encrypted when they are sent: this means that draft messages are not encrypted. So if you are using e.g. offlineimap or mbsync to synchronize with some remote IMAP-service, make sure the drafts folder is not in the set of synchronized folders, for obvious reasons.