In order to give a user a role, you'll need to get his GuildMember object.
To do that, you would have to get the Guild object of the guild you're referring to.
You could simply do that using:
const guildObject = bot.guilds.cache.get('guild id here');
And finally, you could get the GuildMember object by typing:
const memberObject = guildObject.member(userjs); // AKA the user object you already have
Once that is done with, you can simply add the role using:
memberObject.roles.add('791498271922585614');
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…