I am assuming by Entities you are referring to Entidad Urbanística de Conservación. In Spanish we call them E.U.C.'s as the word Entities usually refer to any entity, association, company or community,
These entities have nothing to do with the HPA and are certainly not regulated by them. An E.U.C. can exist even for one family houses that are not regulated by the HPA when there are no areas or installations owned in common by several houses. They are regulated by Urban Laws and regulations and are started by Town Halls when they want to share and regulate the maintenance of public things like roads, sewage, lighting and other services which are normally provided by Town Halls. In their Boards, there is always a representative of the Town Halls. E.U.C.'s are in charge of things like for example, deciding when repairs and maintenance need to be done to public areas that serve an urbanization but are not owned by the communities (as in this latter case, the community would take charge and the HPA would certainly apply).
Each individual owner within the communities (when there are communitites being served by the public installations) or outside communities but whose property is being served by the public installations is considered a member of the E.U.C. The laws and regulations do not clearly establish how to delegate their votes in case of General Meetings. It is, then up to the statutes of the E.U.C. to regulate voting rights and other internal organizational issues.
I believe that Maria de Castro in her reply to the OP did not realize the owner was speaking of an E.U.C. and not of a community regulated by the HPA perhaps due to the use of the word Entity which, as stated earlier, can also be used to refer to a community of owners because it is a word of general use. I believe this because the links she put refer all to communities. E.U.C. are regulated by the Urban Laws as they refer to pubic areas not owned by owner but of service to owners.
If the statutes of the particular E.U.C. are not clear about representation of owners, it is not possible or legal for the President of a community to assume the representation of owners in that community. However, it is debatable where the community has voted by majority to give this powers to the President, whether this resolution of the community binds those owners that did not vote in favour of such resolution. Having said that, in an E.U.C. where a particular community has a much larger number of owners than other communities (as seem to be this case), it is lets say, understandable that the President probably knowing that his owners are not going to bother to attend E.U.C. meetings (as in practice very few people attend) wants to assume the representation of those owners that do not attend the E.U.C. to enable his community to have more weight in resolutions of the E.U.C. than other communitites with a lower number of owners.
Obviously this is understandable but quite possibly not legal and very likely to be successfully challenged by owners in disagreement. However, where owners do not want their President to represent them before the E.U.C., the easiest way out is not to challenge it in Court as this would take a lot of time and money but rather to attend the meeting themselves or appoint one of them as representative by proxy of the others. Their personal attendance or express proxy on behalf of another owner would certainly override the general representation the President would like to undertake.