Everybody in your community will be imported into Savannah as a Member. If you have more than one Source defined, and one of your members is active in more than one of those sources, they will have multiple Identities, one corresponding to each Source.
Savannah will periodically check for duplicate Members, where they have Identies from different sources but a matching full name, username or email address. When it finds these potential matches they will be suggested to you, and give you the option to merge them into a single Member record. When you do this, all the conversations, contributions, notes, tags and other information about the two members will be combined together.
New and Active Members Graph
The Members graph is useful to show you trends about how many members you have coming and going in any given time span. Active members are those who have been made a Contribution or been involved in a Conversation during that time period, and Joined are those members who had their first Contribution or Conversation during that time period. The difference between these numbers represents your recurring active members, those who had joined in a prior period and were still acitive in that later one.
You can filter this view by Member tag to see when people with certain interests or involved in certain projects joined your community, as well as how regularly active they are after joining.
Members by Source
Even after merging Members from different sources, Savannah still keeps track of their identities in those sources and can show you a breakdown of which sources your members are using the most in the Members by Source graph.
If a Member has an identity in more than one of these sources they will be counted once in each of them, for this reason the sum of all of these will be greater than your total number of members.