For the first interview of candidate, do you invite other staff to participate on an interview panel? Who should participate?

Yes, and its probably the division head of which the candidate will be assigned to in case he will be hired. 

It depends on the company recruitment policy. and case by case.

Basically it depends on the recruitment procedures of the orgnization and then it depends on the Job level and the kind of interview if it`s technical, the line manager or a supervisor may participtate if its softskills the HR Specilaist may handle it, the Question is can the HR Specialist judge if this is the best candidate ? sometimes you need an extrenal eye who can help you in the judgmenet, sometimes your role is to get the answers from a candidate and forward it to someone else... there is no one size fits all... but you have to know that having more than one interviewrr may stress the candidate... So if you don`t have recruitment procedures in place you may consider it case by case in order to come up with an effective judgmnet that is fair to the company and the candidate