Do you mean Z2 out of 5/7 (most other platforms) or Z2 out of 3 (Xert/Polar)?
Sweet Spot, Threshold and Polarized Training … By the Numbers – Xert (baronbiosys.com)
I would say Yes you could monitor XSS, ride mostly outdoors, and successfully prepare for your event but it depends on your goals for that event.
Is it a gran fondo/century type ride where you’d like to see how well you do for your personal satisfaction? Or a race you’ll be competing in your class with hopes of placing in the top twenty? 
The former is more of a general fitness goal you can reach by managing TL (doing your own thing as @brianj suggests) where the latter is performance focused that benefits from targeted training (set event date/focus and follow XATA recommendations as @xertedbrain mentions). See third reference link below for details.
When you view your ride activities the table in upper right shows focus/duration and specificity (pure, mixed, or polar). Group dynamics will control the intensity and what focus/specificity results from your group rides. Perhaps you can choose between no-drop C/D group rides versus A/B hammer and drop slugfests. 
In either case if you are not time-crunched you should consider some long solo rides to accumulate LI time (< LTP). Group rides are rarely conducive to accomplishing that.
Solo is also a prerequisite to complete workouts that are practical to replicate outdoors. There are three approaches to riding outdoors – Xert Data Fields on my Garmin - Support - Xert Community Forum (xertonline.com)
Since you’re a newbie I suggest you also review additional tips in this thread –
Beginner questions - Support - Xert Community Forum (xertonline.com)
Reference:
Xert Strain Score – Xert (baronbiosys.com)
Specificity Rating – Xert (baronbiosys.com)
Stress, Focus, Specificity and Strain – Xert (baronbiosys.com)