I posted earlier this month about subscribing to RSS feeds for flickr groups to keep tabs on photos being uploaded.
To locate groups of interest, make use of the search function, located next to the Search field on the right-hand side of every screen. In this case I am looking for groups concerning the Civil War:
The power is hidden in that modest little arrow next to the search. Clicking on this arrow gives you a host of search-refining options. Selecting "Groups" from this drop-down menu gives me some great results:
Note that results can be sorted via the sorting links underneath the number of search results reading "View: Most relevant • Most recent activity • Group size • Date created". I find you'll get the most appropriate results sorting by the default, "Most relevant".