Company contacts are the administrators of the company. They will have access to all the events under that company by default.
Event contacts are managers of that specific event. They will have access to only that event by default.
To give event's / company's an Existing User: access, please follow the below procedure:
Step 1: Login to your existing account
Step 2: Click on 'My Company' tab mentioned on the header
Step 3: Select the preferred company from the drop-down list (in case you have multiple companies)
Step 4: Click on '+Add Event Contact' / '+Add company Contact' and fill up the details to proceed
Refer the GIF Below: