If you are familiar with coindrops, a sharedrop is something similar to that, but what is distributed to participants is shares in the company organising the sharedrop, rather than tokens. There is an important difference though - besides the one of what is distributed to participants - in order for a sharedrop to be compliant to SEC rules.
This difference is on what a participant has to provide in exchange for shares. In coindrops, this is usually some kind of a task undertaken. And this is the reason coindrops are not compliant to SEC rules. SEC sees coindrops as a form of capital investment, but instead of moneny, the participant provides labour. For a sharedrop or a coindrop to be fully compliant to the SEC rules, participants should be able to complete tasks at little or (preferably) no effort.
For instance, if the task a participant needs to complete is to download in his smartphone a free app, then it's SEC compliant. if the participant has to write a commissioned blog post or video on YouTube, this is not SEC compliant (since such a task would require a lot of effort that will take the participant some time to complete).
At ScreenTag, we are combining the two: we are preparing a coindrop that will be a sharedrop as well, since our shares are represented by tokens at a rate 1:1.