- 'Je' and 'jij' are the same, only 'jij' is the stressed variant.
- 'Jullie' is plural.
- 'U' is the formal form (singular and plural).
The formal 'u' is used for instance if you're adressing an older person you do not know and children often are taught to say 'u' to all adults. With regards to the frequency, I think you can compare it to the German 'du' and 'Sie'.