Dear NT:

You wrote:
> Are these sentences correct:
> 1-We have seen people belonging to all age groups.
> 2-We have seen people, belonging to all age groups.
> 3-We have seen some people belonging to all age groups.
> 4-We have seen some people, belonging to all age groups.
>
1 and 3 sound fine. 2 and 4 are incomplete. The comma indicates that the participial phrase is nonrestrictive, but there is no apparent reason indicated in the sentence. They both leave the impression that there is more to be said. That is especially true because of the general nature of the word “people.”