Applying biblical fellowship principles
I ring handbells for a secular community choir and have recently faced difficulties with church fellowship. Many of the ringers are from non-WELS congregations and when their church choir needs a substitute, they often ask us for assistance. Am I correct in saying that acting as a substitute ringer for worship (or performing as part of a small ensemble) at a non-WELS congregation would go against fellowship principles in this case? What about substituting for rehearsals but not worship? And finally, what about playing for a non-WELS wedding at the request of a friend? I suspect that the answer for most, if not all, is that it does indeed go against our fellowship principles, but it would help set my heart at ease to be sure of my doctrinal grounds for refusing to participate.
You have a correct understanding of biblical fellowship principles. It is one thing to participate in a musical group (of singers or instrumentalists) in performance or concert settings and quite another matter for a group to pool their talents in worship service settings. In the latter, doctrinal agreement among the participants is necessary (Romans 16:17; 2 John 10, 11).
When you are not able to participate with other musicians in worship service settings, you have an opportunity to explain biblical fellowship principles to others. God bless the testimony you give with your words and actions!