I understand one may not cut nails on Ch Hamoed if they did not cut them also at the start of YT.
however may I cut them in order to play my guitar for Simchas Chol Hamoed?
And does it make a difference if I’m doing so for a group of people/ tsibbur as opposed to just for myself?
The reason the Ashkenazi Inhag is not to cut our nails if they weren’t cut before Yom Tov is a sort of fine, in order to ensure that we will beautify ourselves before Yom Tov. This includes the fact that cutting our nails on Chol Hamoed is adding to our simchas Yom Tov, nevertheless it isn’t permitted.
There is an option though, one is permitted to cut nails with one’s hands, or teeth. Therefore if it is needed this would be a viable option.