A person who has PS+ can invade your FOB even though you don't have PS+.
They have to face off against your security measures and guards. Since you can't actively join in when playing, you definitely must rely on the security measures and guards alone.
Yes it is possible to change up these tactics and improve your overall defense without having to actively intervene. You can do so from the Mother Base tab in the iDroid, under Security.
There are Basic options and Advanced options.
A good way to form your security is to consider the layout of the deck in question. Under basic, you can determine if you want your guards to be on alert and to what degree. The higher the alert, the faster they run around and actively try to find enemies and the higher the cost to GMP to run at this alert level. Under advanced, you can give them different types of guns between close, mid, and long range (those options only) as well as the number of guards, their stationed positions, security measures like cameras, lasers, drowns, and decoys.
With all these options, you should focus on keeping your security tight, if the deck has wide open areas, give them long range and position them together (same corner), if the deck has cramped spaces, give them mid or close range and spread them out.
Alas, eventually some rivals will infiltrate your base, there's never a perfect defense.