Some commentators believe that this is a symbolic gesture of what was going to happen and Acts 2. Most all of them agree that baptism of the Holy Spirit is a one time deal.
What I believe is actually happening here is that they are being anointed to the ministry they're about to be launched into. The Bible is very clear that only God can forgive sin, and I cannot think of any case in the Bible where any apostle or anyone else forgave anyone of their sins, except for Jesus Christ. If anyone knows an exception of this please post it thank you.
Now are we as Christians expected to forgive those who sin against us, yes most definitely. I like to think that if you forgive somebody for sinning against you it will not be held against them when they stand for judgment.