For coders
Share and work together
There are two ways to get a project in front of somebody else, and they are not interchangeable.
A link anyone can open
Set the project so anyone with the link can open it, and send the link.
Good for a snippet in a bug report, an answer in a thread, or showing somebody something.
An invitation
Invite people by name or email. They appear in the project's list of people once they accept.
Good for work you are doing together over time.
Who you can invite
Only people on your account. The search offers them and nobody else, so if you are on your own account, there is nobody to invite. The link is your route.
To invite somebody properly, they have to be on your account first.
Working in the same project
With more than one person in a project, you see each other's cursors as you type. It is the useful case for pairing on something, and the reason to invite rather than send a link when the work is genuinely shared.
You can also give each person their own copy instead, starting from yours. That suits handing out a starting point, where you want everyone building from the same place without editing the same file.
What next
If what you want is to put runnable code in a page rather than send somebody to the editor, that is an embed.
Share a project covers every sharing option, and working together covers editing at the same time.