firstly, it does of course depend on the group. I imagine that it would be up to any individual group to decide if they'd allow entry to someone whose politics weren't anarchist. Many anarchists will set up non-politically aligned groups to fight single issues within the community and encourage as broad a range of the community to join as possible.
An anarchist group that excludes people who wish to genuinely participate would be a failure.
Re; hierarchies. There aren't hierarchies within anarchist groups. of course there are going to be people who may see themselves as leaders, or people who are basically the leader due to the time and effort they put into the organisation - but decisions are made by the group and ideas are welcome. attendance on actions is not required, if you disagree with something but the majority don't then you are not required to no nor ostracised for not going. Hierarchies are not necessary, and the absence of a formal structure certainly doesn't lead to to "hierarchies running rampant". On the other hand, I would be lying if i said that some anarchist groups have been hampered by this due to having a collection of fevered egos and no leadership structure to prevent them running riot. Most groups I have known have quite adequately dealt with this problem within the group.
Yes, alright, an anarchist group is more like a party than a knitting circle is like a party, but it certainly does not mean that an anarchist group IS a political party. There is a world of difference between the formalised structure of a political party (with a manifesto and a whip and the bizarre interaction between members, activists, leaders etc etc) and the informal nature of an anarchist group.