There are two ways someone such as a client or a contributor can join a workspace:
A direct email invite.
By creating an account from the workspace signup page.
By default, people can only join through a direct email invite (see section 1 below). However, you can choose to also allow people to join the workspace by creating an account through the signup page (see section 2 below).
1) Join through a direct invitation:
Wither you are looking to invite your client or a teammate, first you need to create their profile.
Click on Contacts from the main menu, then click on the New button as shown below:
A popup will appear to create a profile, fill in their name, select their role and tick the invite option to send them their invitation link as soon as their profile is created.
2) Joining without an invite:
You could also allow people to join this workspace without sending an invite manually.
From Setting > Contacts you can enable the option to allow people to join without an invite.