Today() function is not available in the context of a formula based field. The best practice around that is to define a Today Custom Field on object level (in your use case the user level) and create a Scheduled Work Flow to update this fields on a daily basis (on active users, $State='Active').
Nevertheless, in your use case I suggest to create a Chart View (bar chart) on the People module with Login Date as the x-axis. This will allow to view which users (by clicking on each bar) logged on a specific time frame (weekly, monthly, quarterly).
Please let me know if it makes sense.