If you have an office pool from a prior year, you can activate it beginning in June for fall football pools, and January for all other pools (e.g. March Madness leagues, NFL Mock Draft, custom tournament brackets, etc). 

When you log in you will see a message on the home page with a link where you can re-activate your league (you'll see a similar link in your manager control panel):

After clicking this link you'll be taken to the Activate League page:

From here you can select past players to send an invite to, customize the invite email that goes out to your players, then activate the league. You can optionally skip the step of selecting players for invites and just activate your league, then invite past and new players later via the Manager Control Panel 'Invite Players' page.