Junpei's esper abilities would not be able to boost any other character's abilities! While a literal mental connection with any other character would not be allowed--such as what you described with Junpei experiencing another character's exposure effects--it would be fine for Junpei to be intuitively drawn to other espers/psychic characters and perhaps understand them a bit more easily (with the player's permission, of course). No character should have an ability that lets them read minds or access the feelings of others in general.