A user in Second Life is typically called a resident. This term is used by Linden Lab and may be meant to give users a feeling of "belonging" and ownership of the virtual world. "Resident" is also used throughout most of SL's UI in place of "user".
The term "Second Lifer" is sometimes used instead of resident, but not by LL and not in the UI.
Linden Lab's internal style-guide requires the capitalization of the word resident wherever it appears in reference to residents of Second Life.