membership:
Academy membership may be obtained by a competitive nomination (however, the nominee must be invited to join) or a member may submit a name. Additionally, an Academy Award winner who is not yet a member, automatically gains entry into the Academy. The Academy does not publicly disclose its membership, although past press releases have announced the names of those who have been invited to join. If a person not yet a member is nominated in more than one category in a single year, he/she must choose which branch to join when he/she accepts membership.
and nominations: Today, according to Rules 2 and 3 of the official Academy Awards Rules, a film has to open in the previous calendar year (from midnight at the start of January 1 to midnight at the end of December 31) in Los Angeles County, California, to qualify.Rule 2 states that a film must be "feature-length" (defined as at least 40 minutes) to qualify for an award (except for Short Subject awards). It must also exist either on a 35mm70mm film print OR on a 24fps or 48fps progressive scan digital film1280x720. or print with a native resolution no lower than
The members of the various branches nominate those in their respective fields (actors are nominated by the actors' branch, etc.) while all members may submit nominees for Best Picture. The winners are then determined by a second round of voting in which all members are then allowed to vote in all categories.
All information was taken from wikipedia - academy awards.