Quote Originally Posted by Nuta
Please, if you can, explain me some details about translations.

1.Недавно они были хорошими друзьями, но сейчас они видятся гораздо реже.
'Not long ago [until recently] they were good friends, but now they see each other much more rarely.'

2.Мы знаем друг друга с тех пор, как учимся в одной группе.

What sentence is correct?
- We have known each other since we have studied in the same group.
or
- We have known each other since we have been studying in the same group.
Your second sentence is perfect.