The parties in each state choose them. In most states, the state will pick the whole slate of electors for the winning party and they are expected to vote as they are told, so it hardly matters who they are. What you choose when you vote for president is which party's slate your state chooses. Who should then go on to vote 100% for their party's candidate.
There are no members at the moment. The states will choose them on election day in November, and then they will vote on the appointed day in December. The college then disbands as it has done its one and only job and will not form again for another 4 years. College, in this sense, just means a group of people formed to do something and the US constitution doesn't even use the word, it just mentions "electors". The electors never even meet as they are supposed to vote in their own states and the results just get sent to Washington, to be ceremonially counted by the Vice-President on 6 January.