To invite someone to a group you can send invite GROUP_ALIAS USER replacing GROUP_ALIAS with the alias of the group and USER with either the Username, Display Name, an email or a phone number.
Examples:
invite Women4Africa johndoe invite Women4Africa Dr. John invite Women4Africa jdoe@gmail.com invite Women4Africa +5412398734
To learn about the command reference for inviting users into groups your can read the documentation for: