It is a fairly common thing with PD. Soft speech, or slurry speech are motor elements.The answer to this is to overspeak, put more effort into producing the sound even if it sounds loud enough to you it may not to others.

An inability to get words out is a non-motor effect of PD, and as I have discovered from meeting many people with PD, can hide some otherwise very articulate people.

This is a more difficult thing to deal with, and leaves us struggling for the right word, or unable to participate easily in lively conversation, as by the time we have gathered our words the conversation has moved on.

Speech therapy, or specialist methods like Lee Silverman are useful.
Another approach is singing, which can help strengthen, and make more flexible, the whole vocal apparatus. Perhaps it is something the group you mention might like to do.