You can do basic screensharing with Messages on macOS. Open Messages, select a message thread with an existing contact, then from the menu bar, select Buddies >

You can do basic screensharing with Messages on macOS. Open Messages, select a message thread with an existing contact, then from the menu bar, select Buddies > Invite to Share My Screen (or Ask to Share Screen). Doesn't work for groups (last time I tried at least), and hideously undiscoverable.