You'll have to clear Canadian entry procedures to go landside. At least enroute to the U.S., you will have to get your bags (dumped in a special area) as you will preclear U.S. entry procedures at YVR. Not sure if you can meet your friends after going to the transfer desk before Canadian immigration (never had to use it since I live here) and getting your bags to go through U.S. immigration & customs.

On the way home, you won't have to do anything with your bags. You could go landside once you clear Canadian immigration - whether it is advisable depends on what time flight arrival from SEA is.