I'd get on at B19.
B19 is the 79th BP issued for the flight in their sequence, so there there are 78 ahead of you. There may be more passengers than that on board ahead of you though because of through passengers already on and pre-boards or family boarders that got on ahead of you. OTOH, there may be fewer than 78 on ahead of you if they didn't sell many Business Select and some or all of the A1-A15 BP numbers went unused.

The most common Southwest aircraft types have 143 seats, aisle and window account for a combined 95 seats. So, if there are about 78 on ahead of you, there will be some aisle or window seats left, many of those will be next to an empty middle seat. There will be fewer of those left if you wait until B37.

You might even find a row empty boarding with B19. If not, I'd look for a row with one person in it and two empty seats, ask the person if he's saving a seat for someone, if not, take the aisle or window in that row and save the middle for the person joining you at B38. At that point, you're saving a middle seat between two occupied seats and unless you're in row 1 or the exit row, that seat you are saving is no more desirable than all the other middles and you should have no trouble keeping it for the person traveling with you.