A social circle is a group of people connected through casual or close friendships. The circle can be centered around one person (and would therefore be their personal social circle) or around a specific group or activity.
Very often in social circles there is an accepted way of doing things that has developed. New people need to learn the unique characteristics of a social circle before they can be fully accepted into it.
For example, a certain group may meet every Friday night for coffee and to talk about books. If a new person joins, they'll need to meet at the same time and place as the rest of the group, talk books (as opposed to other topics) and generally fit in with the behavior norms established. If they don't, they won't be accepted as readily.
See more: How to get accepted into a group of friends.