Is there any limit on addition of users to the workspace?
Nope. There is no limit on it per se but if you use Users list as a dimension in any of your module then it will have a direct size impact on your model.
As @Misbah mentions there is no limit to the number of users but rarely you would see over 1000 users on one model.
When deciding on the number of users you require it is beneficial to the organisation to decide on an optimum number as there is a license cost associated on a per user basis.
But it is sensible to consider how you would manage a large number of users - user access, defining roles etc.
Also with the new UX it is possible to have users that only have access to the apps so need not ever go into the workspace where the models are.
I hope this helps!