Membership - Changing Payment Gateway from admin to PayPal

(I chose WPMU DEV Dashboard because your plugin dropdown box above wouldn't show me the option of choosing Membership).

I currently have Membership installed (haven't upgraded to Membership 2 yet, because when I tried it a couple of months ago, it created massive problems).

We have a large membership site set up, which we charge monthly or annual fees to access. We often add potential customers manually so they can get a trial of the platform. However, some of them now want to join as paid members.

I tried to move them from payment gateway "admin" to "paypal payments standard," but it didn't seem to work. So that's the first issue.

Then, if I get that to work, how would I actually get them plugged into PayPal? Is there a way to send them an invoice or something so they can log-in via their account and pay?

Thank you!
Sarah Bolton