I am not a SEO expert but, I do not see why that would affect your rankings/crawling. For example, both Facebook and Twitter are high ranking site and each of them have a "splash" page requiring you to login or sign up at their main homepage. 

Ranking/crawling is more based on the volume and quality of content at your site. So the more members, content, sharing, etc. the higher ranking possibility you have when it comes to Google searching. Have 20-50 members where most posts are pictures or just "Hey everyone", you are not going to rank very high not matter what 😁 

Now, if you had your site completely locked down where every page was private and you had a robot.txt file blocking every link, then THAT could affect the ranking/crawling.