Share only with admin, why?

At the moment I can only share folders with users who belong to the “admin” group. Is that correct? I thought I could share with anyone. If I click on share and enter a user name, I get the information that the user was not found, except for those that only belong to group “admin”. Is that correct or what do I have to change? Here user “Test” is not admin, but user. “Test1” is admin. I only can choose “test1”.

Here I can share with users of “no group” without any problem, but I have to enter the entire username. Maybe this is a problem, that test and test1 are too similar?

The problem was the settings in the admin area regarding sharing allowed.