There is currently no way to level sync with a person in your party outside of dungeons/trials/fates, neither would I say to get your hopes up that it'll be implemented since it's an extremely niche feature.

If its any consolation, it's only the unimportant overworld enemies during the MSQ that your level will actually matter with, which 99% of overworld content amounts to basic canon fodder enemies that do nothing but auto and throw an occasional aoe; any dungeons or trials you run will have your level synced.