Basically you can only get invaded if you initiate Co-Op with the beckoning bell. if you never co-op you will never get invaded, aside from some areas that have bell maidens by default like nightmare frontier.
of course, they are slightly different from the ones that only summon enemies, which are found in chalice dungeons and the unseen village post-Rom.
im really not a fan of the bell system for co-op. it feels like a step backwards from souls, where you could look for signs and pick and choose.
it also means if you want a party of 3, you and your summon have to sit around and wait.
also, it can make invasions really rather unfair for invaders, since whenever you invade someone, they will either have a partner, or be searching for one, which leaves the chance of a gank open.
and considering the ONLY online multiplayer aside from Co-op is invasions, (all 3 covenants revolve around pvp in some way, there is no co-op or pve covenant)
It doesnt seem fair.
the covenants are kind of shit too due to the way they handle it. there is no strict structure. In fact, the ONLY covenant that strictly follows the setup of previous souls games, are the vilebloods, AKA the red standard invaders, who have a covenant leader you present invasion rewards to.
the other 3 covenants technically dont have leaders.
the way you enter a covenant, side from the vilebloods, who are the standard "pledge allegiance to the club leader" is by following certain NPC questlines, which nets you a unique covenant rune. they will usually have some special effect, such as stamina regen or healing near death. then they also have a secondary covenant effect. for example with the vileblood's corruption rune, when invading and killing someone, (or killing NPC hunters) you get your designated item to present to the covenant leader.