The chazzan must wait when he finishes a beracha and not start the next beracha,until most of the tzibbur have finished answering amen .
If the chazzan is negligent and does not wait, the tzibbur should not answer amein to that beracha.
Welcoming a new face and conversing with him/her is certainly a mitzvah, and, as such, may be done in a Shul. This only means, however,that it is not sichah beteilah, idle talk. It remains ossur to talk even such talk during davening, meaning when it disturbs others and of course when it is ossur to talk because of Hilchos Tefillah or Hilchos Keri’as HaTorah.
Of course,you always say shalom aleichem, smile, help him/her find a place to sit, and give them a chumash or a siddur.
So as the new faces start coming, say shalom before (and talk),and shalom after (and talk),but (generally)never shalom during.