Grassroots organisations can have a variety of structures; depending on the type of organization and what the members want. These can be non-structured and non-hierarchical organizations that are run by all members, or by whichever member wishes to do something.
To cite a specific hypothetical example, a national grassroots organization would place as much decision-making power as possible in the hands of local chapters or common members instead of the head office. The principle is that for democratic power to be best exercised it must be vested in a local community and common members and instead of isolated, atomized individuals, at the top of the organization. Grassroots organizations can inhabit participatory systems. Grassroots systems differ from representative systems that allow local communities or national memberships to elect representatives who then go on to make decisions.
The difference between the three systems comes down to where they rest on two different axes: the rootedness in a community (grassroots versus national or international); and the ability of all individuals to participate in the shared decision-making process (participatory versus representative.)