This humble plugin shows a tag cloud of all the topic tags for just a single buddypress group. It is displayed above the group forum directory. If You are using the default BP theme it will also show in the sidebar when viewing a single group.
This plugin is still a work in progress. When you click the topic tag in the group it shows you ALL topics from all groups. in a future version this will be changed to just show you tagged topics in the current group.
initial humble release