Answer:
1) Formal, with invitations
2) It is the capital of the Catalonian region.
It is the largest city in Spain.
3) Ellos
Explanation:
1) For example:
Me gustaría invitarle a cenar.
I would like to invite you to dinner.
2) Tibidabo Amusement park is located in Barcelona.
The largest city in Spain is Madrid, not Barcelona.
And Barcelona isn't the capital of the Catalonian region.
3) In Spanish language, it's always a scheme like this:
Ella (she) --> Ellas (them) plural form for she and she
Ella (she) y Él (he)--> Ellos (them) plural form for he and she
El (he) y él (he)--> Ellos (them) he and he