Keywords: joint attention; joint action
That you are aware that your peers are attending to the same target.
AND
That the fact that your peers are attending to the same target is important in you keeping attending to that target.
For a definition of joint action, change all cases of "attend to" to "act on".
Follow-up comment: I thought of the above before reading Siposova and Carpenter (2019). These roughly correspond to what they'd describe as shared attention. Note that these definitions put an emphasis on the expected responsibility aspect of shared attention and imply including discussion of the utility of joint attention into its conceptualization.