Store Info In Database.

I have made a registration form for adding my friends information in a
database. The problem is that I want to connect the persons with companies
in the same database. I wonder how i could connect a person to several other
id's in a table.

I wonder if I can make a string in the database that holds all the id's
separated with , for example....?

the string would then look like this:
1,4,6,22,56
for example...

But how can I then open this string and separate the numbers from the , ???