If you’d like to join a public Pack, click the "+" to either join or request to join (your options for joining are set by the owner of the Pack).
If you request to join a Pack, the owner or one of the admins of the Pack reserve the right to either accept or reject your request.
If a Pack is marked as "Closed," you cannot join. However, you can message the owner to consider your request.
Don't forget you can also create and join your own Packs, too.