Google+ is catching everyone by storm and has already crossed over 20 million users just in 3 weeks of beta release. If you are one of those using Google+ and want to invite some of your Yahoo and Hotmail contacts then you can do the following
Navigate to Circles then click on the Find and Invite link.
Under Find friends, you would see the option for connecting to Yahoo and Hotmail accounts.
On clicking the Yahoo accounts, a pop up would be displayed asking for sharing of Yahoo Profile and connection info with Google.
Enter your Yahoo credentials to sign in to Yahoo. Clicking on the Agree button would start transferring Yahoo information with Google.
Similarly to transfer your Hotmail contacts, click on the Hotmail link.
Then Sign in to your Windows Live Account with the user credentials and this would automatically start downloading the contacts in to your Google+ account. After exporting the contacts from Yahoo and Hotmail in to Google+, you can start sending Google+ invite to the contacts by drag and drop them to the Circles.