Linking table or column key
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.