I have friends offering to invest not insignificant amounts of time in the early stages. They have made clear that they do not want to be co-founders, however, they are offering their time to brainstorm, design, and even some development. I have a feeling I should not accept this without more some at least informal agreement? I am in pre-seed, prototype development stages.
if you're confident go ahead.
By all means take advantage of free help. They are offering to help because they want to see you succeed. Keep in mind that free help isn't always free. Unless you have some type of agreement, they basically can take their intellectual property and do as they please. Have a conversation with them about it. Maybe even draft up an agreement. If the IP conversation becomes difficult or awkward, it's probably better to not accept their help. Difficulty here can be indicative of how they might behave when you have a more formal business relationship or if they are trying to take advantage of you.
They are helping you because they believe in you, so as long as you are confident in your idea, go ahead
" I have friends offering to invest not insignificant amounts of time in the early stages."
Use them to anchor an equity crowdfunding. See:
When you're bootstrapping, you need people like that but be careful on the deal you make and the type of people you make it with.