I am sorry to hear that you feel left out. It can be normal to hang around your friends more or less often. Especially given that it is your second year, your friends are likely to be busier.
I would recommend arranging times to study together, or hang out together. You need not go out with the whole group all the time, maybe just one or two friends at the moment would get you back into hanging around with them. Take advantage of any interesting society events or other events hosted by University that you can attend together.
Hope this helps.
University of Kent Student Rep