Any owner is allowed to vote by proxy. That means the owner on the deed can indicate his desire to allow any other person to vote for him/her.
The number of votes allowed per lot depends on your bylaws. The law states it's one vote per lot but no more two. Our association's bylaws allow only one vote per lot, and explain if multiple owners of the same cannot agree on a position then their vote is not counted.
In regards to a "non-voter" attending and participating in the discussion there are no laws indicting either way. I suppose that could be up to your members to determine. But other organizations/governments often allow non-voters to attend meetings and such. We pass out colored cards, one to each voting member (so a couple would get one card for the two of them) to keep track of the actual votes and prevent double voting or voting of non-members.