In Ingress, can I link to a portal with less than 8 resonators

You have to go there and hack the (destination) portal to obtain the key (yes, you need to hack "friendly" portals anyway to get supplies), or trade keys with team members by dropping/acquiring them.

Also, resonators "drop" keys which are used for links when one end is blown up that you can pick up.

Hack plenty of enemy portals. You get 100 AP for hacking an enemy portal, but none for hacking a friendly one.

If there are no unclaimed portals around (highly likely in some locations), find a level 1 enemy portal and start slugging away. You'll want a few dozen XMP bursters at minimum to destroy the portal, but the rewards are high--destroying 8 resonators and planting 8 of your own on a single portal will get you almost a quarter of the way there. Even if you don't manage to capture the whole portal, you'll still have gotten 75 AP for each resonator you destroy.

Deploying portal shields is a good way to get AP that may not be obvious at first. Check every friendly portal you encounter to see if it has a full complement of shields. If it doesn't, deploy as many as you can. You get 150 AP for each one.