This is not a simple portal topic ...
All the code relating to this is done by SMF. By default SMF cannot show topic titles to guests and stop viewing of the topics. You need a mod to do that.
I run an outdoor club and had the same problem but solved with an SMF mod.
http://outdoorclubjapan.comhttp://outdoorclubjapan.com/index.php?board=39.0You can test it yourself in latest events and those on the calendar. Social events are open to guests.
Mod: 'look but no read'
In admin you set the board IDs which guests can see topics but not open them. This allows the calendar to show event topics (otherwise invisible).
It is an old mod (removed from SMF) but it works the best still for me. You need to make other edits to stop it showing in recent posts to guests and via use of SMF SSI.php. This is fairly simple...
There are other mods on SMF to hide/make private, post details from guests.
One is 'private topic', but works by checkbox for each post. Maybe you can edit so checked by default. There is another board based one that I didnt use as there were too many manual edits or it didnt work well.
I can attach a copy of the 'look but no read' mod if you want it. And the edits required to avoid visibility in recent posts etc.
I also use a Simple Portal custom latest posts and topics by Blue. You can set the number of characters to display from each topic so it is like a 'read more...' teaser. But they need to join the club to see full event details.