make it possible to specify explicit member IDs at group creation time
SSG currently assigns implicit group member IDs using each group members offset in the group view array (e.g., dense ranks). This is simple and works fine for many use cases, but it might also be appealing to SSG users to have the ability to explicitly set the ID for each group member. SSG users who do not want to use dense ranks for member IDs would have to maintain their own mapping from the identifiers they desire to the dense ranks provided by SSG, which just leads to wasted memory and additional complexity.
Allowing users to optionally specify a mapping of IDs -> member address strings at group creation time would be a simple way to overcome this problem. This is somewhat related to issue #3 (closed), in that we need to find a suitable group view data structure that can handle arbitrary member IDs instead of dense ranks. Ideally, the structure allows for members to be easily added or removed and no longer relies on using dense ranks and arrays.