I've recently found a small round hard lump at the top of my groin I can't move it and it's not painful?

It may be a cyst. You should have your physician test it. It may be a cyst? It may be a tumor? or it may be somethign else.. Since it is hard depending on what your studes show such as laboratory tests a dn imaging you may need to have it removed surgically.
Several things. several things could cause this: from ingrown hairs, to epidermal inclusion cysts. Since it is not painful, it is less likely to be an abscess. It is less likely to be cancer, but the only way to rule that out would be to see a physician and have it examined. If it is suspicious, then a biopsy (tissue sample) would be the definitive test to identify it, tho' it's not always necessary.