Assuming that you might be talking about in Telligent Community 6.x -
(a) The setting that enables email digests is a site-level setting - documented in this article.
(b) Email digests are group-level only and can't be enabled for specific applications in the group. Take a look at this article for general information. Here's one on changing your personal email digest settings. As it describes, you can select your frequency in the Group - Banner widget or from your Settings > Email page.
Location on the Group - Banner widget:
Location on Settings > Email tab:
Hope this helps,