Can I specify which fleets can see my site?

Yes, when you create or edit a site, you set under Visibility who can see it. With Visible to all fleets on Depotcharge, every fleet on Depotcharge, BGL Charge and ASTAG charge can find it, while Only visible to fleets with an agreement shows it only to your partners.

Note that visible does not mean usable, as only fleets with an agreement may charge.