Enterprise Edition allows, via request to salesforce, the capability for a system administrator to log in, anytime, as any user. This is tremendously helpful in diagnosing user issues.
But this capability does not exist in Group Edition or Professional Edition. It forces a system administrator to ask the users their login information, and then log in as them. A horribly inefficient process.
There is an Idea posted to fix this. As with all Ideas posted in the salesforce community, users vote for the Idea, and when enough points are accumulated, salesforce considers the Idea for incorporation into an upcoming release.
Please use this link to access, and then vote for this idea.