I can't answer on ASCs specifically. However, buy in occur in pathology and lab too.
You have to understand this is a separate business. You need to evaluate it as such.
Many ASCs are good opportunity but some won't be. Some could even leave you in the hole.
An account and lawyer are essential. Unless your buying now some of this can wait.
You want some assurance you will be allowed a chance to invest when the time comes.
Also, the general terms likely to be offered
When can you buy in? Do you have to buy in?
Are there selection criterion for buy in or this automatic after so much time?
What is your fraction of ownership?
How much does it net per year?
How is that money distributed, i.e. per share holder fraction etc.
Does the ASC have any debt or liabilities? Important !
How is the buy in amount determined?
What was the last buy in amount.
How will your buy in be financed.
Will you have take a loan or will be from the current proceeds of the ASC ?
How long is payments on buy in expected ?