People join a shared space with an invite code — a short 8-character code the owner shares. There is no email invitation to wait on; whoever has the code can join.
Share the code
Open the space and go to Manage. Under Invite code you'll see the current code with Copy and Share. Send it to the person you want to add however is convenient.
How they join
The other person opens Create / join space, switches to Join by code, enters the 8-character code and confirms. They land straight into the shared space as a member.
Regenerate the code
Need to cut off the old code? Press New code. The previous code stops working immediately, and anyone who hasn't joined yet will need the new one. People already in the space are not affected.
Good to know
How many members a space can hold depends on your plan. If you've reached the limit, the app will tell you — see the article on free-plan limits.