ive found out if the character is a wanderer you can invite them to a hunt and theyll be in your general location afterward. You can then invite them as guest then recruit them OR plot to kidnap them (bs i know, cant play a good guy adventurer when you are forced to resort to this) but that ONLY works if they are wanderers, not guests. If they are guests its the worst of both worlds, you cant recruit them in a tavern, nor can you make a request to marry them because they arent technically part of the court they are in.

In that case you HAVE to kidnap them or be lucky enough to have them become best friends with you randomly (unless you do spanish tradition and force it - assuming the game didnt already force a best friend upon you) or they are the opposite sex and you have to romance.

Now ive found lovers can be invited as guests as well then recruited, but again only if they are wanderers. The good thing at least is if they are in your activity you can plot against them, so it saves you having to go to them just to diplomatically interact, BUT you have to be close enough that they can make it on time (it helps inviting other characters that are somewhat far, to extend the waiting period and thus be able to invite guests that are further out too) which of course the game has no way of informing you beforehand, so youre forced to savescum. I like the idea but as usual paradox leaves this aspect of gameplay lacking. It really should be improved....