Inviting people to a call
The host of a call is the owner of its conversation. In a conversation between a provider and a client, that is the provider; in a group, it is whoever created it. The host decides who comes in.
Sending the link
In the call, open the members panel and press Add Members.
Press Copy call link, or Share to send it through your device's share menu.

Adding people yourself
From the same place, pick someone you have been in touch with, and they can come straight in.
Or use Invite by email. Someone without an account gets an invitation that lasts 7 days and takes them to the call once they have signed up. Someone who already has an account finds the conversation in their Messages instead, and if you have not been in touch before, they accept it there before they can come in.
A conversation, and so its call, holds up to 32 people, pending email invitations included. Only a provider can bring more than two people into one.
Asking to be let in
Someone who is not in the conversation and opens the link signs in first, or creates an account, and then sees Request to Join. After pressing it they see Waiting for the host to let you in..., and they enter by themselves once the host agrees. If the host says no, they see Your request to join was denied.
Letting people in
While you are in the call, each request pops up with the person's name and Wants to join. The green button admits them and the red one turns them away. Requests also wait under Waiting to join in the members panel, with a green Admit and a red Deny button, and a red dot on the members button tells you one is there.

Requests reach you while you are in the call, so go in before you send the link. If someone was already waiting before you came in, add them with Add Members.
After someone is admitted
They become a member of the conversation, and can come back to later calls without asking. A conversation between two people becomes a group when a third person joins.
If you turned someone away by mistake, add them with Add Members, in either of the ways above.