[skwdenyer]

If I assign a group to a user, I get a message which says "the assignation was done successfully".

In English, conventionally, an "assignation" is an appointment for a meeting, especially a lover's secret rendezvous. It is true, however, that it is also the act of assigning, but that would not be a usual use.

Might I suggest you update this tag to say "the assignment was completed successfully"?

[breale]

You are absolutely correct!