to summon a group by calling out to them, so that they come together in one place
This verb puts the ordinary word for calling (καλέω) together with a prefix meaning "together," and both halves are doing work. Somebody speaks, and as a result a scattered group ends up standing in one room. That is not quite the same as simply bringing people together (συνάγω), which can happen by any means at all, and not the same as calling a single person over (προσκαλέω), where no assembly needs to form.
Five of the eight New Testament uses are formal. In each one a body with a recognized shape is put into session:
- a governor calls in the chief priests and the rulers
- a high priest convenes the ruling council of Israel
- soldiers call in the whole company of troops
- Paul sends word to the leading Jews of Rome
- Jesus calls in the Twelve and gives them their commission
The other three are domestic. A shepherd and a woman each come home and call in their friends and neighbors, and Cornelius has his relatives and closest friends already gathered when Peter arrives. Nothing official is happening; someone simply wants the people closest to him in the room. In Luke 15 the wording carries the point twice over: he calls them together so that they may rejoice together with him, and the same "together" note sounds in both verbs.
In half the occurrences the Greek adds a personal turn: the speaker is not just convening a meeting but drawing the group around himself. That is why one plain verb covers both a governor's tribunal and a neighbor's kitchen.