Now i have to deal with "Why program takes 34-40% CPU when i click to open Group1, and it was using 4% when on main screen and loginScreen.
Does your code sleep anywhere?
You should use sf::sleep in your main loop to just sleep like 1 millisecond.
If you don't do this then your code could even take 100% of one processor. Why it would suddenly increase I don't know, but if you don't let the program rest for a moment then such things could happen.
If you already use sleep then it is indeed an issue (one that I can't explain).
I can upload entire project if you want to
Not needed. I won't spend my time looking at someone elses code unless I have a reason for it.