When you accept the associate invitation from someone, you will have certain privileges on the team to which you have been assigned.
To be an associate of someone else, all you need is a basic free user account. Being an associate does not cost you anything.
You are welcome to upgrade your own user account, and to even invite associates of your own. That does not affect your status and privileges as an associate of someone else.
The upgraded functions that you have access to as an associate apply only to the posts that belong to the other person's team of which you are a member. Those functions do not transfer over to your own posts in your own user account.
You can be an associate of an unlimited number of other people.
Use Active Team in the menu to switch your posts view and privileges to each team as desired.