command line question

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I have come across something in using bash that really surprises me. I am hoping someone can explain why this is happening. It is not a problem since I know how to get around it, but I would like to understand it. It goes like this: 1. open a new terminal window 2. verify who I am 3. display groups. Note that one group is missing from the list 4. switch users to myself 5. display groups. Note that all groups show now. I noticed this because I could not access a directory that I should have had access to based on group membership. ()