You should realize that you don't have to choose from two sentences: you can make up one that is not ambiguous:I think sometimes it's not so clear cut which one to use:
"Those who would like to sing a song will be welcomed/welcome." (coming from a host inviting people to sing at a party)
This is how I interpret the two meanings, and I may be wrong:
"...will be welcomed (by the people there)"
"...will be welcome (to do so)"