First of all, it's good you took up the challenge to help your friend with performance - you will definitely be curious to look at the piece that your comrade created. The author of this piece also deserves some words of praise for being brave and putting herself on the line. To play the pieces created by real master is of course very pleasant but you will get an entirely different feeling when performing your own composition. Besides, playing together with a friend, you will surely have this thought: "I think I could do something like this, too. If she has done it, perhaps I also should try."
Obviously all of the advice in this article will be valid if you play pieces by other composers for chamber ensembles.
It might well be that you are pretty confident on the piano but still new at the organ - perhaps you haven't had much experience playing different instruments in public. Perhaps the pedal part of this piece is not very challenging (a composition for Communion can't really be very virtuosic), but as you prepare for its performance, you will have to face 3 main difficulties:
1. To figure out which stops you want to use (because the author of this piece probably isn't an organist herself and didn't indicate the precise registration.
2. To adjust the stops you want to use to the real situation you will have during this church service.
3. To rehearse the piece so well that all 3 of you can perform it with a peace of mind.
It shouldn't be very difficult to figure out the registration of the piece, if you are familiar with the organ and its construction. Most often in an ensemble we use a few flute stops of different pitch level (8' and 4'). For the pedal part, if you play the lowest part with your feet, choose 16' and 8' flutes.
In case there are some places in this piece when the organ part needs to emphasize some melodic voice in the soprano or tenor, it's nice to assign this part a separate manual with a solo registration.
Usually when we play in ensemble with other musicians, we have to keep in mind the loudness level of other instruments. If the flute plays a solo part in this group, then surely stronger reed stops (Trompette, Posaune, Bombarde, Clairon etc.) will be too much for the soloist.
Of course, during the church service reeds must be chosen carefully. During Communion the music should have a meditative and prayerful character. Therefore, even if you chose some kind of solo reed stop, it would be best if it was rather soft - Oboe, Clarinette, Vox Humana, Krummhorn etc. For the fullness of sound 8' flute used together with a reed works very well.
But you also have to be prepared for the scenario when your desired reed will not sound in tune (this often happens). In this case, I suggest you use something like 8', (4'), and 2 2/3' flute stop combination.
If there are instruments which sound stronger and louder (such as trumpet or trombone), you might want to add some principal stops to the mix. Principal 8', perhaps with the reinforcement of the Octave 4' would create a suitable balance between organ, solo instrument, and voice.
When you have already thought through the concept of the registration of your piece, please study the target organ also. If you haven't played this organ before, contact the local organist and ask for an opportunity to come and see the place or get the specification list and photos.
Maybe you will find out that this organ only has one manual and there will be no possibility to use any solo stops; maybe this organ doesn't have pedals; maybe you will notice which stops are not working or are out of tune.
Of course, it would be best that you found out the state of the instrument and its capabilities before you agreed to play. Then you wouldn't need to make some major adjustments in the score and the piece would sound as well as it could.
Now there is only one but perhaps the most important part of all left - to rehearse with all 3 performers. This is crucial and as you will find out, it will make or break the performance. I don't even need to remind you how vital it is that every musician would know her part very well - this is the matter of professional attitude. Additionally, every musician in your group would do well if they were familiar with each other's parts. Then you wouldn't play separate parts independently but would create a real musical conversation.
It would be best if you could get at least one rehearsal beforehand on the target organ. But make sure you do your homework first - figure out and write in the registration in the score.
Whether to write in the numbers of the stops or shortenings of the manuals and stop names it's entirely up to you but the latter way is much safer, preventing you from accidental errors when you draw or write in the wrong stop number. If you have all your registration down, then you will save much precious time which can be used at the organ for actual music making.
When you make music together, always keep one ear open for the echo in the church itself - this way you will know exactly how the listeners will hear your piece. Only keeping this in mind, make choices about stops, articulation, and tempo.
By the way, all 3 of you have to rehearse ahead of time (either at another organ or piano). While rehearsing, always remember that you will have different keyboards, different sounds, different balance between performers, and different acoustics.
Organist must react at once to these circumstances. It's best if you can guess how it's going to be and play ahead of time in the similar way at the organ where the performance will be. It's not easy to achieve. Your earlier experience might prevent you from appropriately imagining the sounds of the new instrument. Also the skills of piano playing might come back to you again and again.
But don't worry - the earliest steps are difficult for everybody and the feeling of insecurity is normal. Remember - the more organs you will know, the easier it will be for you in the future to adjust. Therefore I recommend for you not to stop learning and drive across your area and try out as many diverse (large and small, old and new) instruments.
[HT to Egle, Migle, and Emile for inspiration. They will perform a "Prayer" composed by Migle at the Cathedral of Vilnius during church service tomorrow.]