Joining a Team plan is done by invitation. The Pro subscription owner will have to add you as a team member. You will receive the invitation by email. 

If you have an Iconfinder account, accepting the invitation will directly give you access to the Team plan. 

If you don't have an Iconfinder account, you will be asked to create one before getting access to the Team plan. 

More about Team plans: 

How to leave a Team plan
​How to add team members to your Iconfinder Pro plan
​How to remove members from your Team plan