Let's say there are two people who are friends with each other. (Platonic friends). Maybe they're even best friends. But they feel like they always just hang out with each other instead of expanding their social circle. So they want to make new friends, maybe only one or two new friends because they're introverts and value close connections with a few people. Ideally they'd like someone who could be friends with both of them and all hang out together.
They would never say "We're looking for someone to join our friendship!"
They don't say that, because a "friendship" isn't something that can be joined. Just as a relationship, or a marriage, isn't something that can be "joined."
Friendships and relationships are FORMED between two entities, not JOINED by an extra person.
You can join a "group of friends," not a "friendship." You can join a club or a business or an organization. You can't join a relationship.
You can FORM a relationship with someone, or even with two someones, or three someones. But you can't JOIN a relationship or be added into it.
I would be more sympathetic to unicorn hunters if they said, "We are looking to re-form our marriage as a triad with a third person," rather looking to "add a third" to our wonderful, perfect marriage.
Another problem I have with the phrase "join our marriage" is that it sounds too much like "join us in bed." Joining someone in bed is fine, joining a couple in bed is fine, joining a circus is fine--but you can't join a marriage.
Single, straight, female, solo, non-monogamous.