Upgrade Modules

Hi,

I am in the process of coding a module. I have seen that some modules alter base tables in drupal. I would like to know if I for example add a column to the users table, if I were to upgrade drupal, would this then reset the base tables to defaults?

reson I ask, I have groups that different users connect to, I need to add a column in the users table that will have an id of the group that this user belongs to.

Will the columns stay?

Thianks in advance,
Davin