If your customers are having trouble logging in, you can easily change their password from your side and send them the new credentials. Here's what you'll need to do:
Click on the Audience tab, at the top meny.
Find the customer you're looking for and click on their name to open up their profile.
Click the three dots in the upper right hand corner, and choose "Edit customer" from that drop down:
Here you can change their password or even correct their email if they used the wrong one or misspelled it:
Hit "Save changes" and you're done!