Weekly challenge invites friends AUTOMATICALLY in the private group, not the random group.

I joined the weekly challenge with one of my friends in the group( not the random one). The intention was to finish the challenge by the two of us. But the game automatically invited another person from my friend list. Neither me nor friend invited anyone to the group. This happened 3 times. It happened when I invited certain friends to the challenge, but the group has less than 5 people.

It isnt invitation, people can join the group of a friend anytime, it was that since the weekly challenges.
Later they add invitations, but the feature that people from the friendlist of any player can join to the group continue at it is.

Or they have the envelope of invitations? One have to be at least one in the group and if the others have, some player have to invite them.

For us, like a year ago, a player that doesnt contribute, invite to completly fill the group each time he join, he go away.
One time we dont do anything until saturday afternoon, he have 1k flowers and steps, there was an limited event in that week; another time he do the invitations and our IRL friend cant join because of that (since that time we invite her).
We have enough friends for helping them without parasites like him (we finish planting challenge in 1 day and depending of the rest the 100k steps in 2-3 days)

But then is there a way to keep the weekly challenge group at only 2 people? I don’t want other people on my friend list to join. I didn’t invite the third person, and somehow he just showed up in the group.

Yes, you can invite non-active players to close the group, like that player do with our groups. Dont know why, we ask him and no answer, that the reason for delete him.