I am begginer in Zazen. I can not physically sit in lotus or full lotos so I am sitting in virasana on my knees. Does that influence quality of Zazen? When I sit together with other people I feel not so comfortable mentally because I am the only one in group who can't do even half lotus...

If you can sit Zazen on your knees, that is great. Actually sitting in any lotus posture is easier than sitting on your knees. What is more important than the form of sitting is how you hold the concentration in your mind.