How to understand this sentence with 次第 and 締め切らす

Question

I am not sure I understand this sentence,

定員になり次第、締め切らせていただきます。

In particular, I can't figure out what is omitted.

Here is what I understand,

As soon as you will become a regular member, I would like to close (something)

I did a quick a search on Google and this set phrase seems to be really used a lot. In what context is it used?

Answer 1

定員{ていいん} as described on wikipedia means:

組織・団体などの場合、定員とは組織・団体に属しうる 最大数の人員 のことを指す。

or

施設・設備・自動車・鉄道車両・航空機等、ある区画の中に人を入れる場合には、定員とはある一定の基準の下にその区画内に入れる 最大人数 、またはその目安という意味がある。

The key point being the maximum number (note the 最大人数 and 最大数の人員 I emphasized). In other words, 定員になり次第 means As soon as the maximum number of people is reached. The reason you noticed its used a lot, would be this would be used often for public events where there is only a limit amount of seating, etc.