If you're planning on using dreamhost mail there is no easy way, add them one by one.
If all 58 of these employees will be using a common Internet IP or a large number of them do (as offices typically do) I believe you might trigger a dreamhost security mechanism. Normal traffic from a single ip and traffic from 58 employees sharing a single ip are two different things. The ip might get banned (I've seen threads here that suggest that, but to be honest I don't know what the specific trigger is.) You might have luck communicating with support on this, or you might not.
Also FWIW, dreamhost does not have business class email. Prepare yourself for complaints. Prepare to pull your hair out solving email problems. (Or not solve them actually, all you can do is open a ticket and let the customer vent towards you.
Dreamhost does WEB hosting very very well. They suck at email tho (they've admitted this in the forum, although that post appears to have gone missing). My advice to you is don't put 58 of one of your customers on dreamhost email. Unfortuntunity what I do recommend isn't free and costs per user, but it's a business class solution that you won't get complaints about.
Read this forum, there are many many threads here that will reinforce what I just summed up for you.