Never email passwords or other confidential information. Use LastPass, or another top-rated password keeper such as OnePassword, to send passwords.
Email is not a good way to communicate passwords. Email servers get hacked, and email transmissions get intercepted. You can set up encrypted emails, but that takes some work for you and the recipient (another topic).
With LastPass, one of the things you never have to do again is “email someone a password”.
Better approach with LastPass is to click their icon to Share a password. When you share a password, you can specify whether the other person can ever see it or just can have LastPass fill it in for them. You can also store and share notes, account numbers, etc. via LastPass.
A better approach for WordPress is for you to create a user. You specify what permissions they should have (for example, Editor or Author, or if they truly need it Administrator). You email them (to their email address you used when creating the user) the URL to your website/wp-admin/. When they try to log in, they click the “Forgot password” link and make their own password (using, of course, LastPass to generate it and store it).
Want to make a good Master Password for your password manager, that is extremely difficult for hackers to guess, yet easy for you to remember and type? Read my Strong Passwords for WordPress .