If you wish to invite friends to a Circle that are already part of your other Circles, tap Invite and then "Invite Existing Users."

Users who are in the largest number of Circles with you are displayed at the top.

You can search for specific users by display name and filter by Circle membership.

Tap Invite to send, and the users you selected will receive a notification in app. They will have the option to either Accept or Decline the invite.