"Attendance at lectures, meetings, training programs and similar activities need not be counted as working time if the following four criteria are met: (a) Attendance is outside of the employee's regular working hours; (b) Attendance is in fact voluntary; (c) The course, lecture, or meeting is not directly related to the employee's job; and (d) The employee does not perform any productive work during such attendance." from http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_785/29CFR785.27.htm
So from 3-5pm, they must be paid based on (a) alone and those hours would go into the overtime calculation. And I would really question criterias (b) and (c) as to whether they should be paid from 5-7pm.
The DOL goes on in Section 785.28 to state "Attendance is not voluntary, of course, if it is required by the employer. It is not voluntary in fact if the employee is given to understand or led to believe that his present working conditions or the continuance of his employment would be adversely affected by
nonattendance."
I would also ask the goal of the employer of having this seminar during/after regular work hours. Did they want better attendance than doing so say on a Saturday? Are they getting some type of a premium break on business insurance? If so, they may have made it less voluntary by having it (partially) during work hours. And unless the non-paid totally voluntary training was communicated that way in advance to the employees, I would not try to argue it now...because it would take just one person making a wage claim stating that boss "told them they had to go" to cost more than paying them.