A client is one organisation in VSTA, such as Ledgerline, with its own map and its own members. Each client has an address of its own, such as https://vsta.work/ledgerline. You only see the clients you have been invited to.
One client
If you belong to one client, VSTA opens its map as soon as you sign in.
More than one client
If you belong to more than one, VSTA shows Your clients after you sign in. Choose a client's name to open its map.
To change client later, go to the VSTA address with nothing after it, such as https://vsta.work. That opens Your clients again. You can also keep a bookmark for each client's address.
A facilitator also sees New client under the list. Create a client explains it.
Nothing listed
If the page says "You do not have access to a client yet", nobody has invited you to one. Ask the facilitator who works with your organisation for an invite.