Hi Anil,
From your post your points are - i) whether a Union(representatives/office bearers) can enter the office/other premises and ii) in the matter of negotiations & settlement/agreement with whom the management has to interact.
There is no restrictions as to who can form the union(s). Any group of employees can form union subject to the conditions stipulated in the act. It could be either with affiliations or can be independent as well, no compulsions. And registration has to be obtained from the ALC of the relevant jurisdiction. These regd.formations should obtain the recognition of the management and can function independently or with unison on matters for common benefits of employees and/or taken up matters concerning welfare of employees. In order to avoid confusion law permits, there shall be only one recognized union as the single formation duly elected amongst all the regd unions for the given period. While doing so they need to discuss with various officials of the management. Though management is within their prerogative to discuss only with the elected & recognised union often it couldn't restricted only with the elected one for obvious reasons. Like political/social and/or with a view to maintain rapport with all formations for future developments. So you'll have the right to deny entry of unions to those are not elected/recognised unions, provided you'll have every tactic in your armor to tackle the situations wrought in a particular moment. As a shrewd HR moderator, you'll know in the course of your tenure whom to receive warmly and whom to avoid for the moment in a way fitting within your constraints.
With regard to settlement & enter into agreement on issues delibrate, you are duty bound to sign the settlement only with the elected/recognised union. Sometimes major issues., long terms settlement such as wages, leave, incentive schemes, bonus rationalisation of manpower, welfare measures etc. would call for consensus of all regd.unions in which case the agreement can be signed by all the regd. unions and involvement of ALC as a tripartite agreement is a welcome strategy of the management. How you'll bring all the regd.unions on one platform is the secret of success of any HR team and the wisdom of management.
I can cite for example of labour/IR policies of TATA group for others to emulate. I have not heard of any TATA estt (having their presence in almost all sectors, world over) stalmated with labour troubles in modern era, Uno-WB project which ultimately shifted to Gujarat notwithstanding.