For Form 5500 count for a welfare plan, what is a participant ?
Is it someone who is eligibile for plan or who is also actually benefiting?

The definition for welfare plans is different than that for 401(k) plans, etc. I don't have it in front of me -- but I think that the general conclusion is that it only applies to those who have enrolled in benefits.

I believe the Form 5500 instructions provide a fairly detailed discussion of the definition as well including reference to the definition provided under DOL Reg. § 2510.3-3(d). Note also that dependents of employees participating in welfare plans are not counted.

Yes I took a look at this DOl cite.
It seems to be saying anyone eligible but unlike 401(k) I thought otherwise, that you would not include anyone who didn't enroll?