Are your last activities visible to everyone?

Yet to add to what ​@Jan_Mantau said, also the map must be visible (can't be hidden), and it doesn't work for manual activities either. Here's a very comprehensive article with additional details: https://support.strava.com/hc/en-us/articles/115001631264-Inviting-a-Friend-to-Your-Activity

Edit to add: if I’m looking at the correct Strava profile (two running sessions from today, one of them with a photo with your mates), it looks like it's really because of the map settings - the map is visible on your previous runs but it's hidden on your today's activities. Make the map visible and you should be able to add the friends. 

Go to the activity and click the Add Others button.  This will let you add other people to add your activity to their records and thus willl have done the activity with you.  But do remember they have to accept it themselves, you can’t force it to them.