You only need to create one user account, no matter how many associations you are a part of.
Link multiple associations
When you need to link to an association that is NOT the FIRST one you need to be connected to in Unioo, simply click the invitation link and log in with your usual login details.
Switch between associations
When you need to switch between your associations within Unioo, follow the steps outlined in this help article: Switch between your associations.