If you want to add an annual or monthly fee for your members you will first have to order this at Support.
If you want discounts then you can include the 'membership group' in the regular automated discounts as they can be used here as well. Discounts are described in their own FAQ.
The automated sign-ups (and billing) begins on the start date you set in the membership activity. So if you set January 1st everybody is enrolled from that date.
The time span (start and end date) you set in the membership activity controls the addition of membership fee when a customer enrolls in an activity that's running within that time span - So it has nothing to do with when the customer signs up for an activity! Ie. You set the membership start date to 1/1 2019. A customer signing up for an activity running during December is not added to the 2019 membership. Another customer signing up for an activity running during January is added to the 2019 membership.
Once this is set up it runs automatically for both super-users and self-service via your online booking