What can cause swollen lymph nodes in groin/neck?

It depends. We need to know more . How long have you had these nodes and what is their size? Are they increasing in size? if they are bigger than 2 Cm and increasing in size in 1 to 2 months, you need to be examined by a physician. If the nodes are suspicious for Cancer, a biopsy is the best way to find out or exclude Cancer as the cause. Infections can also cause swelling of lymph nodes(Tender to touch)