Not sure where question belongs.

Illinois
We as a company do a large amount of purchasing from outside suppliers. When the supplier offers a golf outing perk to all the mid management group, can someone from within the group purposely "not invite" one member, simply because he/she does not "like" that person?

The supplier has no obligation to invite anyone at all, and can choose who his guests will be. Unless the reason he "does not like" the person is BECAUSE OF their race, gender, national origin etc. there is no illegal discrimination.

I should have been a little more specific. It is someone from our company dictating to the supplier not to include certain people. Not the supplier.

It doesn't change the answer, though. It only changes who would get the complaint filed against them IF the excluded employee has a VALID reason to believe that it is their race, religion etc. that is the triggering factor.

Someone on your management team excluding you from the outing because of personal animums (i.e. they don't like you) is NOT prohibited discrimination.

Can I ask what you mean by "prohibited" discrimination?

Certainly. Prohibited discrimination is that which is forbidden by law; i.e. race, religion, national origin, gender, disability, pregnancy, or age (at the Federal level, over 40 only). Some, but by no means all, states have added a few other groups to this listing; in Illinois the state additions are citizen status, arrest record, marital status, military status, or unfavorable discharge from the military.

So if the individual is being excluded BECAUSE OF one of the reasons above, it is illegal. If s/he is being excluded for ANY OTHER REASON WHATSOEVER including that the decision maker simply does not like them, it's not.