I have always absorbed new members at any time they apply and get accepted.
The principal times of influx are, of course, when the season starts in late August or early September, and, to a lesser extent, after the New Year.
But, I have never turned those who passed muster away.
They always seem to fit in and adapt easily, and the older members are glad to have them.
Two times i absolutely refuse to receive new members are a week or a month before Christmas and a week before Easter or during Lent.
It is notable that some thoughtless people just want to come sing for Christmas or Easter and then they disappear.
A choir is a choir.
One is either faithfully in it from Transfiguration to Corpus Christi, or unfaithfully not in it.

There are only one or two times during the season where I ask new joins to wait... after the start of Advent or Lent as MJO mentions above. In those cases, I ask them to wait until after Christmas and Easter respectively - because there is a lot of material to absorb if they are coming mid-stream. But for the rest of the year, I'd rather get someone integrated rather than forcing them to wait for some arbitrary start time.

I don't want to inflict a lot of note bashing on the current members

I don't find that practice will change because there are a few new people. There is a constant cycle of "new" and "old" material - for those that have been with the group from the start of the season as well as those who join later. I provide play-back files for people to use on their own outside of practice, and I encourage people to sing intelligently (i.e. not being a Pavarotti when you don't know the music as well as others in the group - or even when you do!).

I don't want to scare the new people off with an avalanche of unfamiliar material.

If I see someone is getting overwhelmed, I'm very reassuring with them. I've compared joining the choir for new members to drinking from a fire-hose... but honestly, someone who is new to our repertoire is going to feel like that whether they join at the beginning of the season or mid-stream... because there is a constant cycle of "new" and "old" material as above.

There are some who will curse and revile me for saying this... but the music is not the end of itself. It is simply the means to the end. I want people to realize the spiritual joy of full participation in the Liturgy, of drawing souls to Christ through the medium of music. If our focus was merely on the performance (a show choir, say), then I would handle new additions differently, maybe once or twice a year. But as a stand-in for a true liturgical choir, we have a different purpose. We strive for perfection, but the real measure is in the effort toward that perfection, not the accomplishment of it per se.

There are some additional thoughts to try and overcome the second concern. The first is to check in frequently with the newer members... making sure they feel welcomed by the group, making sure they understand the resources available to assist, making sure they understand that I'm not setting an arbitrary standard on when they'll be fully integrated with our repertoire. It may take a longer time for some and a shorter time for others.

Also, I highly recommend having "choir angels" who mentor new members. This can be assigned if necessary, but you may find that the choir is pretty responsive in that regard on their own (my groups tend to be).

Good luck with your new members, whenever you choose to admit them!

Musicians who miss rehearsals with the group must miss singing the polyphony at Mass. Singing hymns with loud organ is one thing, but adding excellent un-integrated singers on the day a piece is scheduled is asking for trouble.

I have a new bass who joined us just before Christmas. He already knows the Kyriales, so he's not entirely silent; he sits with the schola, and when we sing the more complex polyphony, (so far) he follows the pages. And, for some of the pieces we work on at practice, he's singing.
All the other singers give him pointers and encouragement, without ever being asked to do so. I asked him about a month ago about drinking from the fire hose :-) and he said it's a wonderful experience. Good positive attitude.
I agree, it's key that the CD must remember the purpose of the choir is drawing souls to Christ through the medium of music, as Incardination said, but this also needs to be the new singer's reason for joining Choir. I find that anyone who joins for an artistic experience, or for camaraderie, will not stay.

I think in such cases it matters a lot whether the singer is knowledgeable and can learn quickly and how much extra energy is required from you to get them up to speed. For example, I only accept new singers into our “Mini Schola” (which is for 7-12 year old beginner singers) once every 2 years. They require a lot of work and energy because they must be trained from scratch. On the other hand, our advanced group can accept new singers at any time because they are advanced. Anyone joining at that level doesn’t need much help. Sometimes with new people who are not advanced enough, I just bring them in on certain songs so they aren’t overwhelmed with new material. I certainly cannot have them learn what everyone else already knows but it’s easy to bring them in on certain pieces. Having singers at different levels of ability is a big challenge though. This is why we have 4 distinct groups for our one TLM. Minis, Juniors (middle school age), Advanced trebles (teens and up) and Men. They sing different things at times and other times they all sing together. It’s a juggling act for me as their director, but overall it’s a nice variety of types of choral music (children chanting angelically, full SATB polyphony, men chanting, etc)

In this day and age in Québec, one cannot turn down new vocations! Since I can sight read Gregorian notation, and since I have extensive experience with the Ordinary Form liturgy and Liturgy of the Hours (we *only* do OF... not by ideology, but because there's no local EF), the choirmaster places newbies next to me. He asks that they sing in a very low voice or not at all at their first liturgies until they get the hang of it, but lets them work in rehearsals. I help get them through the finer points. Fortunately most newcomers we've had lately have had some choir experience, albeit not Gregorian.

Ora