To define membership to members you have to create a membership plan and then register as a vendor with the membership plan from the front end.
At present, you cannot assign the membership to your already existing test vendors on your site from backend.
From the backend the option is not available because there are some terms of membership plan and the renewal of the membership etc. To have those things automated the manual assignment is not available via admin panel.
In that case, you have to create a separate plan with the same capabilities and make sure to uncheck the “Show in the membership list” option (https://prnt.sc/kznp3u) and set the plan as Free. This will not display this membership in the plan page.
Now, share this plan link with the users. Once they will register under this, they will be assigned under this membership.