You can invite users to Scrapinghub projects by going to the members page:
And entering their email address:
An invitation will be sent via email where they can click a link and complete the Scrapinghub signup process. After they have signed up, they will become members of the project automatically.
Please note that project members have restricted permissions -- they will be able to deploy and run spiders, but not use Scrapy Cloud addons (Crawlera, Splash).