In Google+ you can increase your friends by searching your friends accounts by clicking your friends accounts.and also Google+ suggest some of the friends which you may know at the right side of the page by adding them you can increase your circles after searching you can drag them in to the circles which you want like friends,family etc.
It is better if you explore your need before increasing the number of people in your circle. You won't get benefit if there are only so many people in your circle, but they give you nothing...
Make sure that your friends or people add you into their circles. If you update any status (this can be used for marketing), so they can see you.
good idea from himanuzo... you better build your circle with your friends and acknowledges first. If everything goes well, you can use your circle as your marketing media...
Increase the amount of friends. Your friend who are now using G+, add them. Who are not using G+, invite them. Add friend from the suggestion list. Do everything naturally.
To increase your Google circles you must divide your circles in to different circles and add few people suggest by Google+ in your account.
make your profile public, that way you will be viewable to everyone on the web.and always post interesting content.through this work people will like you content and you can increase your circle.
If you are a marketer then Don't increase randomly. Increase your desired Customers as your friends because it will give you more conversion rate.