Hi there !
Joining the skate is completely free, no obligations, no strings attached. Just show up a bit before 10pm at the meeting point and join the fun!
Only obligation is good behavior during the skate, and obeying the directives given by staff members (no skating on sidewalks, etc...)
For rentals, your best bet is Nomades :
http://www.nomadeshop.com/They are located Place de la Bastille, right where the sunday afternoon skate (
http://www.rollers-coquillages.org ) starts.
On the Nomades website, click on "location" (means "rental" in french) on the left-side menu (sorry, direct links don't work). Rental for 1 day is 9euros, the full week-end is 15euros.
You'll have to get your skates on friday evening before they close (7pm).
Beware : they don't take credit card payments for skate rentals, and they require that you leave a piece of ID (passport) with them for the duration, as deposit (or a 150euros check, but probably only a french one).