Linking table or column key Simple structure: 

have a table PAGES - which stores information about specific landing pages
working on developing a GROUPing concept for the pages, where we are saying that a page may or may not belong to a group, and it will only belong to a single group.

Would a linking table group_page.group_id,group_page.page_id be the proper way to go?  Or
pages.group_id ?
This is rather subjective based on the nature of the data, potential for change, deadlines, longevity of the project, storage space, performance requirements, etc.  

However, the determining factor will likely be the probability that you will ever need multiple groupings for pages.   If you will, then implement a join table now.  If not, putting Group_Id inside the pages table is faster to build, less complex, will perform better, and uses less space.