Here's some wording you can use -
"The guidance we’ve been receiving from Alchemist is that it will be too late for others to join once the institutional lead comes in. These institutions have a history of taking the whole round when they come in.
Given that, we are doing a close now for smaller checks we want to make sure can be involved. We want to do this now in advance of the institutional lead.
It is best if you can invest now as we can’t guarantee there will be room later.
If not, we will aim to hold $250k for you in the round if an you let us now:
(1) Is there a minimum of other capital you want to see co-investing with you in this round?
(2) We may opt not to have one institutional lead who will crowd everyone else out. And instead, a syndicate of investors so we have a plurality of voices amongst our investors. If the only way we could get a path for you to invest is via a syndicate, would you want us to still hold space for you?"