I don't have a g+ acc yet, but I was curious if for example I have a couple of friends on G+, can other friends viewing my profile see all the friends I have?
On your profile, you can set it either so that they see people who you've added to a circle ('friended'), people who have added you, or both, or neither. The default setting is both.
I thought I'd add some additional clarification. For people you've added there are two sharing concepts: Who you're sharing with and what you're sharing. For who you're sharing with there are 3 options: no one, everyone on the web, only the people in your circles. For what you're sharing there is the ability to pick and choose what circles are visible. It might be confusing to think about, but suppose I have the following circles: "Family", "Friends", "Coworkers", "Poker Buddies". I can choose to share the friends in these circles to everyone on the web, but opt to exclude "Poker Buddies" so they don't show up on my profile.
For people who've added YOU to their circles, there are only two options. Show them on your profile or don't show them.