So you are extending Sales Cloud and you want to test or show your work off to a few colleagues. Sharing new extensions between developers or administrators is easy. Just have them join the sandbox where you developed your extensions. But sharing sandboxes with end-users seems not to be possible. Typically their role definitions will not allow them to do so, or can they?
There is a way for you to grant them access to your sandbox though. They cannot join your sandboxes themselves, but you can include them in your sandbox by setting a profile value on their user levels.
Using the ‘Setup & Maintenance’ task : Manage Administrator Profile Values, after having searched for the profile option ‘Active Sandbox’, you can set for any user what sandbox they should get access to n order to test your extensions.