Once you move beyond the generic “add connections” option that LinkedIn has, you might want to specifically search for and increase your connections with people aligned with whatever connection policy you might have.
The challenge is that restrictions exist within LinkedIn that may prevent you from inviting others you don’t know personally. You are entitled to try to connect with people without knowing their email address, but once five people respond to your connection invitation by saying they don’t know you, your ability to connect will be restricted.
Once you’ve decided to connect with professionals who aren’t part of your network, chances are good that you will initially find them by doing advanced people searches. If you are already an experienced user, you’ll probably encounter people you might want to connect with everywhere on LinkedIn.