Been meaning to reply this thread and honestly in my opinion, the best way to invite your friends to the Key Universe without spoiling it is to let them discover it themselves. Maybe mention about one of the anime series and what they’re about every now and then when they ask what have you been doing for fun, and if they are interested, say “I recommend checking it out”.
On the other hand if they’re not interested then it’s best not to force the series on them and let them discover it themselves. Cause if you force any of the series on them, there’s a good chance they might not like it and it might cloud their judgement towards the other Key works. And as @Yerian said, Key is not really for everybody. There are some who can’t stand their works and they have their own reasons so you can’t necessarily stop them from feeling how they feel. The only thing you can do is let them discover the works themselves and if they don’t want to or they don’t like them, that’s fine. Can’t win them all.
With that being said, one of my short term goals is to let my Mom see Clannad and Clannad After Story one day. ^^