I haven’t done this with instruments yet. But I did just do this with singers last month. I needed a choir part and found some people who wanted to sing on my project. I also sang on it so I sent them my mix with my own vocal part. I actually put a lot of different mixes in a Google drive they could all access. I did a full mix with my vocal, full without, separated stems, a click track, and kind of a narrator track where I just recorded myself doing a count in for each section they needed to record. When they were done they put their recordings in the Google drive so I could download them and add them to my mix project.
On the one hand it added some extra wait time where I couldn’t really do anything on the project until I got their recordings. On the other hand it gave me an outcome better than I could have done alone and it was a lot of fun.