I speak for a group

Someone announces that they speak for a group because they SAY they are part of that group.

The claim "I speak for a group because I SAY I am part of that group" presents a few issues in terms of logical fallacies, cognitive biases, and reasoning errors.

### 1. **Fallacy of Assertion (or Ipse Dixit):**
The assertion that someone speaks for a group purely because they claim to be part of it is an example of the Ipse Dixit fallacy, which roughly translates to "He himself said it." This appeal to authority lacks evidence and provides no supporting argument that the person actually represents the views of the group. Just saying something is true does not make it true without corroborating evidence or logical reasoning.

### 2. **Bandwagon Fallacy:**
Claiming to speak for a group without any verification might be an attempt to leverage the bandwagon fallacy. This fallacy occurs when someone asserts a belief is true or valuable simply because it is endorsed by a group. Just because someone claims group membership does not inherently validate their authority to represent the group's opinions.

### 3. **Confirmation Bias:**
The speaker may be exhibiting confirmation bias by selecting information that justifies their claim while ignoring evidence to the contrary. They might assume that their own views are naturally representative of the entire group's opinions, without actually checking if this is the case.

### 4. **Logical Gaps:**
There is a logical gap or non-sequitur in the reasoning. Even if a person is genuinely part of a group, it does not necessarily follow that they are authorized or have been chosen to speak for the entire group. Membership alone does not confer representative authority.

### 5. **False Authority:**
By claiming to speak for the group, the individual might be creating a false impression of authority. Unless there is a formal mechanism within the group where members elect or designate spokespersons, this claim lacks legitimate validation.

### Conclusion:
To robustly claim the ability to speak for a group, the individual should provide evidence of:
1. Being authentically part of the group.
2. Having been designated or elected as a spokesperson by the group.
3. Representing the views of a majority or significant portion of the group.

Without these, the assertion remains logically weak and could mislead others about the actual views and representative processes of the group.

I think it's just a non sequitur. There's no logical connection between the two statements.

Consider

1) I am a woman

2) Therefore, I speak for women.

What's the relationship between 1 and 2? There isn't one. To speak for a group means that you represent the views held by its members, but if the group is sufficiently heterogenous (like women in the above example) then simply being a member of that group means that your view is just one of many, which indicates that you do not in fact represent the group.

Institutions might have spokespeople who are given the responsibility of speaking for a group. But they will be much more homogenous than a group based on mere identity. And even then, the spokespeople will often be told what to say, rather than giving an opinion and claiming it as that of the institution.

Assuming that this person is being truthful about being a member of the group, I could see this being used in one of two ways. 

One is that the group is small. Maybe you have spoken to every person in your department at work, or on your soccer team, and you can truthfully say that every person in the group agrees with what you're saying. 

Another is when you're talking about a large demographic group. For instance, "I speak for men when I say beer is better than wine ." It's highly unlikely that the person has spoken to the entire group, or that there is a consensus. In that case, the person would likely be relying on their own experience, that of people they've met or seen in the media, etc. In which case they'd likely be committing the hasty generalization fallacy.