Q: Is there a difference between the Site Administration password and the Group Administration password?

The site admin and group admin passwords are different.

There is exactly one site admin password, which gives access to the entire site. In particular, it will let the user change various options like the text on the main page, e.g. where it says "Welcome to ProjectForum" or the blurb beneath. This can all be done in the "Site Admin" area, accessed via the bottom right of that page. This is also where the site admin password can be changed. The site admin is also the only person who can delete groups.

There is one group admin password per group (so if on your site you have Group#1,#2,#3 - each would have a group admin password. To sign in as a group admin, you'd enter the group and then click where it says "Group Admin" near the bottom right of each page. This is also where the group admin password for that group can be changed.