Hello Jean,
It is possible to invite other users to join your sandstorm server, and they would have the ability to create grains within any of the applications installed on the system, yes. The interface to add full users is in the Admin panel, specifically at (your sandstorm URL)/admin/users/invite
Additionally, it is possible to share access to certain grains with other users using per-grain “Share” settings, and (in many apps, not necessarily all apps support this) you can control whether they can edit or just read the grain.
I hope that answered your question. Hopefully you can hop in the demo server (linked in the guide above) or spin up your own instance and give that a try.