Con?
Congregate: Con - with, together; Greg - to flock; ate - to cause to become
To cause to become flocked together.
Construct: Con - with or together, Struct - to build
To build together.
I never said anything about "con-", I was speaking about "cog-". The prefix "con" is very useful to know, just like "co-". I was wondering whether you could think of many examples of "cog" used as a prefix to mean "together," to corroborate the inquiry of it being a useful prefix to know.